Police fear a summer of disorder as warm weather and the easing of coronavirus lockdown restrictions create a “perfect storm” for trouble.
Officers are braced for a surge in violence when pubs in England reopen next Saturday, dubbed “Independence Day”.
The warnings follow a series of mass gatherings around Britain which flouted social-distancing rules as the sun shone and temperatures soared this week.
Dorset Police declared a major incident after thousands flocked to Bournemouth.
A mass brawl broke out among youngsters at Ogmore-by-Sea, South Wales.
And 22 police were hurt when an outdoor party in South London turned violent.
Experts feared the gatherings could unleash a fresh wave of Covid-19 infections.
As the UK death toll rose by 186 today, reaching 43,414, Boris Johnson warned people against "taking liberties" with social-distancing rules – fearing it risked a "serious spike" in coronavirus infections.
Speaking during a visit to a restaurant in east London ahead of lockdown relaxations on Monday, the Prime Minister said: "If you look at what's happening elsewhere in the world where people have been coming out of lockdown, I'm afraid what you're also seeing is people taking too many liberties with the guidance, mingling too much, not observing social distancing.
“So in some parts of the world – I won't name them – you have got spikes, really serious spikes, in the instance of the disease, so it is crucial that people understand that on July 4 we get this right, we do this in a balanced way."
His comments came after Health Secretary Matt Hancock threatened to close beaches if distancing measures are ignored as thousands flocked to the south coast on the hottest day of the year.
Pressed on whether the Government would shut beaches, a No10 spokesman said: "It is for local authorities to manage numbers.
"I believe Matt Hancock, when he spoke, was referring to the powers to impose localised lockdowns which – we have been clear throughout – may need to be the case if we see case numbers rise in a particular area."
The spokesman went on: "Everyone should be able to enjoy the sunshine, and we understand that people want to enjoy public spaces, but it is important we don't undo the hard work of the British public in reducing the transmission of this virus.
"It is a matter for local authorities to manage numbers, alongside emergency services and Public Health England.
"We have been clear that should we see case numbers increase we will introduce local lockdowns."
The PM was urged to set-up a coronavirus war room to deal with large gatherings when the lockdown ends.
Former Defence Minister Tobias Ellwood, the Bournemouth East MP who chairs the Commons Defence Committee, asked for councils to be given more help to deal with incidents as restrictions are eased.
He told the BBC: "I really would urge the creation of a national situation centre that can monitor events across the country.
“Don't forget on July 4 we're going to add alcohol to this equation as well, and I would hate to see Bournemouth or any seaside resort become that place where the second spike is the first to appear.”
England's Chief Medical Officer Professor Chris Whitty urged people to follow social-distancing rules in the hot weather or risk causing a spike in coronavirus.
He wrote on Twitter : "Covid-19 has gone down due to the efforts of everyone but is still in general circulation.
"If we do not follow social distancing guidance then cases will rise again.
"Naturally people will want to enjoy the sun but we need to do so in a way that is safe for all."
Ian Hudspeth, of the Local Government Association, said: “It is understandable that people want to enjoy the sunshine and councils are working hard to reopen public spaces and find safe ways to help people enjoy good weather this summer.
“However, lives depend on all of us taking personal responsibility for our actions.
“Some of the scenes we have seen in recent days have been unacceptable.
“When large numbers of people head to the coast there is inevitably a risk of overcrowding and an increase in the rate at which Covid-19 could be transmitted.
“It is hugely difficult for a council to keep beaches open to the public while also limiting the number of people who visit. Closing beaches in one area may also simply shift the problem elsewhere.”
Police fear lockdown-weary Britons venturing to the pub for the first time in more than three months when bars reopen in England next weekend could drink too much and, in some cases, turn violent.
Police Federation chairman John Apter said: "I'm really concerned.
“The summer time always adds extra pressure.
“People are on holiday, they are out drinking, they want to enjoy themselves.
“What we have this summer that is different is the build-up of pressure over months of lockdown, the added frustration of worrying about the virus, people are losing their jobs."
Mr Apter claimed the lockdown relaxations and the reopening of pubs meant the country had been "sold countdown to carnival”.
He said alcohol, sun and frustration created the "perfect storm", adding: "We as the police have got to be prepared for everything."
Former Metropolitan Police Commissioner Lord Blair feared the public was becoming “complacent” about violence towards officers, following widespread trouble this week in Brixton, South London, and at Black Lives Matter protests earlier this month.
He wrote in a letter to The Times: “My sense is that we are becoming complacent about assaults on police officers.
“There needs to be a consensus that this is not acceptable.
“I am not asking for changes to sentencing but more for a shared sense of public and political outrage.”
Metropolitan Police Commissioner Dame Cressida Dick promised to shut down illegal parties after officers came under attack for a second night in London as they tried to break up crowds.
Dame Cressida said the force have a "duty" to stop unlawful music events during the Covid-19 pandemic and vowed: "We will be prepared this weekend."
Her comments come after officers sent to break up a street party in Notting Hill, west London, were pelted with objects on Thursday night.
Dame Cressida said police were on the lookout for illegal parties after violent scenes in Brixton on Wednesday evening left 22 officers injured.
She added: “We have seen some large numbers of people completely flouting the health regulations, seeming not to care at all about their own or their families' health and wanting to have large parties.
"It is hot, some people have drunk far too much, some people are just angry and aggressive and some people are plain violent.
"We will be prepared this weekend.
“We have officers all over London working hard again to try to keep the peace and to protect our public from violence and disorder."
