LONDON (Reuters) - London mayor Sadiq Khan pleaded with residents to stay at home on a sunny Sunday, saying the advice to avoid socialising was meant to save lives and police could get involved if people ignored the warnings.

FILE PHOTO: Mayor of London Sadiq Khan attends the ceremony of unveiling a plaque informing about financial support of London to the Auschwitz-Birkenau Foundation during the ceremonies marking the 75th anniversary of the liberation of the camp and International Holocaust Victims Remembrance Day, in Oswiecim, Poland, January 27, 2020. REUTERS/Aleksandra Szmigiel

Britain has ordered gyms and bars to shut, and said restaurants should only offer takeaway meals. But with no enforced lockdown in place, many Londoners went out to enjoy the fresh air in parks and markets.

Mayor Khan said that although exercise and dog-walking were a “good reason” to go out, anyone planning to meet up with friends should think again.

“Life has changed. We’ve got to do things differently for a while now. Social interaction leads to the disease spreading, leads to people dying. Don’t leave your home unless you really really have to,” Khan told BBC television.

“This isn’t advice as far as I’m concerned. These are instructions and these are rules that we should all obey to stop people dying.”

People were out walking and exercising in southeast London’s Greenwich Park, though work-out groups were spread out over the grass as they tried to keep their distance.

Families were also out in outdoor spaces from Columbia Road Flower Market in the east of the city to Battersea Park in the west.

But just over the River Thames from Battersea, the borough of Hammersmith and Fulham said all its parks would close from 1900 GMT on Sunday until further notice, the first part of the capital to announce such a move.

The National Trust, which manages historic properties across the country including London, also closed its parks and gardens from Sunday.

Under emergency legislation announced last week, the quarantine powers of police will be strengthened, and Khan suggested that these powers could be used to tackle Londoners who went outside without a good reason.

Asked if police could stop, fine or send people home, Khan said: “Clearly, if it is the case that people continue to act in a way that’s leading to this disease spreading, then those sorts of things will be things that will have to be considered.”

“Be under no doubt: by mixing, it leads to more people dying,” he said. “Please please please, stop mixing and stay at home.”