U.K. prime minister Boris Johnson has declared a partial lockdown of the country, effective immediately.

In an address broadcast to the nation on Monday evening, Johnson urged Britons to stay home to save lives. The prime minister forbade leaving home for anything apart from shopping for basic necessities like food and medicines, medical needs, one form of exercise a day, and travelling to and from work for essential services only.

Gatherings of more than two people are also banned. The police can be pressed into force to keep people off the streets and to ensure the social distancing essential to prevent the spread of coronavirus.

Only shops providing essential services will remain open. In addition, parks will stay open, although gatherings of more than two will be prohibited.

“Without a huge national effort there will come a moment when no healthcare service can cope,” Johnson said. “It is vital to slow the spread of the disease.”

Monday’s heavy-handed measures — which will remain in place for three weeks — are the result of the British public’s failure to heed the government’s advice. On March 20, Johnson ordered all cinemas, theaters, bars, gyms, cafes, restaurants, pubs and clubs to shut down and renewed his appeal for people to stay at home and venture out only if essential.

However, this past sunny Spring weekend saw thousands of Britons ignore the prime minister’s advice and throng to parks and open spaces in the thousands. On Monday morning, the London Underground was packed with rush hour crowds flouting the advice to stay home.

“The way ahead is hard, and it is true that many lives will sadly be lost,” Johnson said.

Meanwhile, U.K. foreign secretary Dominic Raab has asked all Britons abroad to return home immediately, while they still can. The latest travel advice states: “Any country or area may restrict travel without notice. If you live in the U.K. and are currently travelling abroad, you are strongly advised to return now, where and while there are still commercial routes available. Many airlines are suspending flights and many airports are closing, preventing flights from leaving.”

The U.K. has 6,650 confirmed coronavirus cases, with 335 deaths to date.