Local authorities at England's Brighton Beach have asked people to stay away after crowds flocked to the nation's coast on the third-hottest day ever recorded in the United Kingdom.
The Met Office said the 37.8 degrees Celsius recorded at Heathrow Airport made Friday the hottest day of the year so far in the UK.
Brighton and Hove City Council urged people to not travel to the area, saying they were concerned about the number of people already in the city and that it was impossible to maintain social distancing due to large numbers.
Meanwhile, further west on the coast in Dorset, police and council officers were patrolling Bournemouth Beach and the seafront at Poole, with large crowds amassing despite social distancing guidelines being in effect.
Dorset Council also urged people to avoid popular summer spots such as Durdle Door and Lulworth as car parks had filled up.
In June a "major incident" was declared at Bournemouth after similar crowded scenes sparked COVID-19 fears on what was the UK's previous hottest day of the year.
In Scotland, police were called to a large disturbance at Edinburgh's Portobello Beach as temperatures hit 28C in the capital.
Police Scotland said there were no reports of injuries and officers were still trying to establish the circumstances of the incident, but police were seen confiscating alcohol from beachgoers.
UK Prime Minister Boris Johnson reiterated warnings about the threat of a second wave as he announced England would be delaying the easing of some restrictions due to a rise in the infection rate.
Mr Johnson gave the warning a day after a late-night announcement of a new lockdown in northern England and said the reopening of casinos, skating rinks and bowling alleys had been delayed, and wedding receptions would have to be cancelled.
"We're now seeing a warning light on the dashboard," Mr Johnson said.
"Our assessment is that we should now squeeze the brake pedal in order to keep the virus under control."
The changes, combined with the stricter lockdown imposed on more than 4 million people throughout Greater Manchester, east Lancashire and parts of West Yorkshire, are the largest reversal of restrictions to date in the UK.
England's Chief Medical Officer, Chris Whitty, said the Government had probably reached the limit of reopening society and the economy without causing an increase in the number of infections.
"We all know that what we have to try and do is to get to the absolute edge of what we can do in terms of opening up society and the economy without getting to the point where the virus starts to take off again," he said.
More than 45,000 people in the UK have died from COVID-19, according to Government figures.ABC