Jurgen Klopp says they are "dangerous", Eddie Howe says they could cause a "tragedy" - pitch invasions and fan violence have dogged English football in recent days as the season reaches its climax.
Since Monday, there have been five pitch invasions, players assaulted, Crystal Palace boss Patrick Vieira clashing with a fan, arrests made and investigations launched.
According to the Football Offences Act 1991, it is an offence for fans to enter the pitch "without lawful authority or lawful excuse".
So why has this happened, what is being said and what will happen next?
What has happened?
- On Thursday, Crystal Palace boss Patrick Vieira was involved in an altercation with an Everton supporter during a pitch invasion following his side's Premier League defeat at Goodison Park
- On the same night, Swindon Town players were "physically and verbally abused" after their League Two play-off semi-final defeat by Port Vale
- The other League Two semi-final, played on Wednesday, also ended in a pitch invasion with Mansfield boss Nigel Clough saying such incidents are "getting concerning" after his player Jordan Bowery appeared to be shoved by a fan late on in their win at Northampton
- Earlier on Thursday, a fan was jailed after running on to the pitch and headbutting Sheffield United captain Billy Sharp at the end of the Blades' Championship play-off semi-final shootout defeat by Nottingham Forest on Tuesday.
- On Monday, Luton boss Nathan Jones said Huddersfield supporters were an "absolute disgrace" following their pitch invasion.
Swindon forward Harry McKirdy said on Instagram: "I'll take the stick and the songs. But bottles, coins, lighters thrown [at] me, running on and hitting and kicking me/team-mates. Too far."
And his manager Ben Garner said: "I don't know where we're going to as a country, but it's absolutely disgusting."
'We want to avert potential tragedy' - what the managers said
Klopp's Liverpool side could win the Premier League title on Sunday, but he believes celebrations can take place without pitch invasions.
"It could have been dangerous. I'm not sure how you avoid that," he said.
"I don't want to judge. I understand emotions but for the other team it's dangerous. I really hope we learn from that. We should make sure absolutely nothing happens.
"We can celebrate things without threatening ourselves and the opponent."
Newcastle boss Howe, who takes his side to relegation-threatened Burnley on Sunday, said: "I don't mind the celebrational aspect - embracing the success that a team has had is part of football, I've got no issue with that.
"It's the aggression towards the opposition, it's swarms of people around one or two people.
"That doesn't sit well with me at all, that's something [on which] we have to act very quickly because we want to avert potential tragedy."
Norwich manager Dean Smith said more needs to be done to protect players' safety.
"I don't think security-wise we're doing enough about it," he said.
"Football fans, we missed them during the Covid period, but come on, give your heads a wobble, you don't want to be running on and attacking people and managers.
"In general society you can't go and abuse, verbally or physically, anybody on the street but for some reason you're allowed to do that at football. It is a major concern at the moment."
Arsenal's Mikel Arteta said: "We have to stop it and minimise the risk and exposure of players and staff in this situation because you cannot control it. When you cannot control it, with so many people involved it becomes dangerous."
'This cannot continue' - FA
The FA said it is "very concerned about the rise in anti-social behaviour from fans".
In a statement on Friday, the FA called on individual clubs to "play a vital role" and prevent pitch invasions as well as "taking their own action". The FA said it was "reviewing our regulations to help stamp this behaviour out and to ensure the safety of everyone inside a stadium".
The FA added: "Football stadiums should always be a safe and enjoyable space for everyone, and these incidents are completely unacceptable and have no place in our game.
"It is illegal to enter the pitch area in any stadium and these actions are putting players, fans and people who run the game at great risk. This simply cannot continue and we can confirm that we are investigating all of the incidences."
Why has this been happening?
The pitch invasions have come at the end of the first full season with fans present in stadiums since the start of the Covid-19 pandemic in March 2020.
Geoff Pearson, a professor of law at the University of Manchester who researches crowd behaviour, said the effects of the lockdowns has caused a "deterioration in the behaviour of some fans", adding this has affected the relationship with fans and police, who "rely on knowing who the fans are, including the potential troublemakers, and all those opportunities during the pandemic dried up".
He told BBC Radio 5 Live: "Once we had the first major pitch invasion then there is a certain element of copycat, because there is a certain element of fans wanting to be seen as authentic.
"That's one of the real challenges - what is authentic fan behaviour?
"Unfortunately, at the moment, running on to the pitch, setting off smoke bombs, is what is seen as what authentic fans are doing."
In February, the national lead for football policing, chief constable Mark Roberts, said cocaine use is helping to drive a rise in disorder at matches.
On Thursday, the government announced anyone caught in possession of or supplying class A drugs in connection with football faces a five-year ban and their passport being taken off them.
Police data released to BBC Sport in January showed arrests at football matches across the top five English leagues are at their highest levels in years.
A Professional Footballers' Association spokesperson said: "Since the return from lockdown, police data shows that incidents of fan disorder have been on the increase, and this has coincided with a rise of assaults on players by fans.
"While the broader issue of increased fan disorder requires a coordinated and long-term strategy, the clubs, leagues and police need to put their immediate focus on stopping large-scale pitch invasions.
"Encroachment on the field of play is against the law. While the majority of fans may be celebrating, it is clear that for a minority, this behaviour becomes toxic, aggressive and dangerous. This scenario has resulted in an unsafe environment for players and club staff. People are at risk of being seriously hurt or worse. The football authorities need to be tough and send a clear message. These are predictable events, it is a criminal offence, and it is unacceptable."
What have others said?
After the incident involving Sharp, the PFA called for a review into matchday security and fan disorder, which was later echoed by Preston North End manager Ryan Lowe.
He said the violence seen this week in football is "disgusting" and "bizarre".
"Fortunately it'll get stopped for next season for the simple reason that Billy Sharp has had to take the nut from some idiot, got stiches in his face, because it could have possibly killed the lad. It cannot happen ever again," he said.
"Who is to say Billy Sharp wouldn't have banged his head and never woke up?"
Writing on social media, Accrington Stanley chairman Andy Holt said it is a "disaster", adding: "You win, you draw, you lose. If you can't cope with that STAY AWAY.
"You're ruining the day for many, increasing club security cost and as a result ticket prices for all. I'm not paying extra costs because of these stumps.
"If costs go up, tickets go up. Ban them all, jail them all, we don't need them in football."
However, Everton boss Frank Lampard doesn't see pitch invasions as an issue - so long as fans behave.
"If it is done in the right way let them stay on the pitch, let them have their moment," he said. "As long as they behave, no problem."
Chief Constable Roberts said the rise in the number of pitch invasions was "alarming", calling on fans to "celebrate in the correct way".
"The pitch is the players' place of work and like everyone else, they should be able to feel safe," he said.
What happens now?
As the fan who headbutted Sharp has discovered, the repercussions of such behaviour can be severe. He has been jailed for 24 weeks, fined and given a 10-year football banning order.
In March, an Everton fan who threw a plastic bottle at Aston Villa players celebrating a goal in their Premier League meeting in January was given a suspended prison sentence.
Pearson believes "a lot of legal action will have to be taken against a lot of people" in the close season as fans, particularly those who have been involved in violence during the pitch invasions, are identified.
Those issued with football banning orders are prevented from attending games for at least three years, and can be asked to surrender their passport to ensure they do not travel to matches abroad. Those who breach the orders face six months in prison.
Chief Constable Roberts told BBC Radio 5 Live that a return to having perimeter fencing at stadiums is not the answer, adding it would be "a failure of us all".
The Premier League, EFL and clubs are receiving advice from the Crown Prosecution Service on how to build the strongest cases possible against fans who attack players.
On Wednesday, the EFL said it was to "consider what further measures are now at our disposal" in order to tackle crowd behaviour.
On Friday, the Premier League said it is "concerned about fan behaviour at grounds", but that "reckless actions are carried out by a small minority of fans and the majority of supporters attend and watch matches in a safe and respectful way".
"The Premier League has encouraged clubs to remind supporters that it is illegal for them to enter the pitch at any time and may result in a criminal record and stadium bans. We will continue to discuss and address the ongoing issue of fan behaviour with The FA, EFL and other football partners ahead of the start of next season to ensure matches are safe environments for all."
The FA does have the power to order ground closures - either full or partial - for issues involving supporters, while fines have previously been issued to clubs.
The two most significant fines in recent times were West Ham being fined £100,000 when fans went on to the pitch during a game against Burnley at London Stadium in 2018, and Aston Villa being fined £200,000 in 2015 when their fans repeatedly went on to the pitch during an FA Cup tie with West Brom at The Hawthorns.
Birmingham were fined £42,500 when a fan went on to the pitch and punched then Aston Villa player Jack Grealish during a derby game at St Andrews in March 2019.
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