Prime Minister Boris Johnson has urged the British public to abide by social distancing, with some parks saying they will close if Government advice is not heeded.
Johnson has reminded the public that fresh air does not provide immunity against coronavirus and that people should stay two metres apart.
The Prime Minister told Britons on Sunday that open spaces like parks are important for people's well-being but they should follow social distancing rules and take government advice "seriously".
"It's crucial that people understand that tomorrow the schools are closed," said the PM.
"I want, of course I do, people to be able to go to the parks, open spaces and enjoy themselves.
"It is crucial for health, physical and mental well-being.
"But please follow that advice and don't think that fresh air in itself automatically provides some immunity.
"Take this advice seriously, follow it, because it's absolutely crucial."
There have been warnings that some major parks in London could be shut if social distancing advice is not followed, amid large numbers of visitors.
The Royal Parks, which maintains those parks and five of London's other largest open spaces, posted on Twitter: "After scenes of crowds not #SocialDistancing yesterday, we want to urge you again to follow the government's advice and keep a safe distance from others.
"We have a shared responsibility to keep London's parks safe for everyone who needs them.
"We will keep this situation under constant review. If people do not follow social distancing guidelines, we will have no choice but to close the parks."
Hammersmith and Fulham Council have already shut their parks from 7pm on Sunday "until further notice", while Kew Gardens has been shut "as a precaution to limit the spread of coronavirus".
Victoria Park and Columbia Road flower market in east London were also pictured as being full of people on Sunday.