British Prime Minister Boris Johnson on Monday night announced strict lockdown measures for the UK in response to the coronavirus pandemic, which has killed 335 people there.
In a televised address, Mr Johnson said that gatherings of more than two people would be banned.
"From this evening I must give the British people a very simple instruction: you must stay at home," Mr Johnson said.
He said Britons would be able to leave the house only to shop for essential food and household items, one form of exercise a day and any medical needs.
"That’s all. These are the only reasons you should leave your home," Mr Johnson said.
He urged people not to meet anyone who does not live inside their house.
"If you don’t follow the rules, the police will have the powers to enforce them, including through fines and dispersing gatherings," Mr Johnson said.
The government will also close shops that do not sell groceries or essential health items.
Under the rules, other areas including libraries, playgrounds and outdoor gyms, and places of worship, must close immediately.
The government is also stopping all social events, including weddings, baptisms and other ceremonies, but funerals will be allowed.
Parks will remain open for now so people can exercise once a day.
Mr Johnson said the rules will be reviewed in three weeks, with an aim of relaxing them if possible.
He made the announcement after chairing a top level emergency meeting of senior ministers, medical advisers and scientists, which was conducted by video conference to avoid the risk of spreading the virus.
"Without a huge national effort to halt the growth of this virus, there will come a moment when no health service in the world could possibly cope, because there won’t be enough ventilators, enough intensive care beds, enough doctors and nurses," he said.
“In this fight we can be in no doubt that each and every one of us is directly enlisted.
"Each and every one of us is now obliged to join together, to halt the spread of this disease, to protect our NHS and to save many many thousands of lives.
“And I know that as they have in the past so many times, the people of this country will rise to that challenge. And we will come through it stronger than ever.”
Some Britons were criticised over the weekend for not taking instructions on social isolation seriously enough.
In Wales, Mount Snowdonia had its busiest ever day on Saturday, in a sign that members of the public were not complying with the rules.
Parks and beaches across the country were full of people, prompting a backlash and criticism of the government for not being strict enough.
On Monday, some trains were still packed with commuters.
Monday's announcement follows the government's move to close all concert venues, gyms, restaurants, pubs and theatres on Friday.
Until now, the British government was reluctant to implement the strict self-isolation measures that some countries in Europe brought in.
But Mr Johnson has taken a tougher line since coming under pressure from MPs and public health experts.
Earlier on Monday, Foreign Secretary Dominic Raab said that Britons travelling abroad should return home as soon as possible because of increasing complications with international travel.
