Video report by ITV News Correspondent Rebecca Barry
The Prime Minister has warned people against "taking liberties" with social distancing rules as he said it could lead to the danger of a "serious spike" in coronavirus infections.
ITV News Science Editor Tom Clarke analyses the mass gatherings and what they mean for coronavirus
It prompted heath secretary Matt Hancock to warn the government will take action to close beaches and public areas if people flout safety restrictions.
Boris Johnson says the public must "stay alert" as England's lockdown eases:
Speaking during a visit to a restaurant in east London ahead of an easing of England's lockdown on July 4, Boris Johnson 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."
Despite Mr Hancock’s warning of possible beach closures, Downing Street signalled such a move would be a matter for local authorities.
The PM narrowly avoided shaking hands with a well-wisher on a visit to a restaurant in east London on Friday:
Labour leader Sir Keir Starmer said the "government has lost control over messaging" following scenes of "overcrowding" on the country's beaches.
"I’m really worried about the scenes we’ve seen from some of our beaches with overcrowding," he said.
"This dates back to Dominic Cummings and that affair, where essentially the Government had a message that the public read as one rule for them, another for us."
The Labour leader was referring to the controversy sparked by Mr Cummings' 260 mile drive from London to Durham during stricter lockdown measures.
Sir Keir added: "The Prime Minister has to take responsibility for this."
He urged Boris Johnson to "up your game on communications because this is very serious".
The council said extra police patrols had to be brought in following the "irresponsible" behaviour of crowds who gridlocked roads, dumped rubbish, abused refuse collectors and parked illegally.
Dorset Police issued a reminder to beachgoers on Thursday that the country is "still in a public health crisis" - with packed scenes on the area's beaches.
Bournemouth East MP Tobias Ellwood said Dorset had seen "half a million visitors" on what was the hottest day of the year so far.
ITV News Political Correspondent Libby Wiener on the government's reaction to the crowds:
Police and security were seen patrolling the beach on Friday morning, although the crowds had not arrived in such numbers, possibly put off by forecasts of storms later in the day.
Local resident Pat Munday described how cars were double-parking and blocking driveways - preventing emergency vehicles from getting through - while people were using the gardens of her apartment block as a toilet.
The 73-year-old said: "It was just mayhem, people just want to have a good time but they just don’t give a monkey's about others.
"It's a shame, it's such a beautiful part of the country but people just need some control, people have lost all sense of respect - when they urinate in your garden, it's the pits."
Vic Williams, 75, said: "There was no idea of social distancing, when we left at 2pm the crowds were still arriving, there wasn't any two metres.
Because the restaurants are closed, there's nowhere else for people to go."
Mr Williams said the council's tractors had been out in the early hours clearing up the tonnes of rubbish left behind by sun-worshippers.
He added: "The litter afterwards was unbelievable, they just walk away from the beach and leave their rubbish, there were tents left, BBQs, it costs us ratepayers a fortune."
Local resident Sandra Harthill says emergency services 'could not cope' with the volume of people:
A senior lifeguard said there was an "excessive" number of people during the hottest days, and added: "Bournemouth has a reputation as one of the best beaches in Europe [...] but it's important we work together so everyone can feel safe."
Teenager Jessa Bane travelled down from the Reading area with a group of friends to enjoy the beach after finishing college.
The 18-year-old said: "We have created our own social bubble by drawing a line in the sand around us to make sure we can social distance."