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London’s mayor pleaded with residents to stop “all non-essential use of public transport” Tuesday, as pictures emerged of crowded subway cars, just hours after the United Kingdom ramped up coronavirus prevention measures.

The government is now ordering most stores to close, banned gatherings of three or more people and said everyone apart from essential workers should leave home only to buy food and medicines or to exercise. Citizens were reminded of the "new rules in force” through a text message that the U.K.’s mobile networks sent out Tuesday, but it appears to have not stopped them from packing the London Underground transit system.

"I cannot say this more strongly: we must stop all non-essential use of public transport now,” Mayor Sadiq Khan tweeted. “Employers: please support your staff to work from home unless it's absolutely necessary. Ignoring these rules means more lives lost."

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The government has said police will have powers to break up illegal gatherings and fine people who flout the rules. But some are expressing doubts about whether or not they can be enforced.

"There is no way really that the police can enforce this using powers. It has got to be because the public hugely support it,” Peter Fahy, former chief constable of Greater Manchester Police, told the BBC.

Prime Minister Boris Johnson announced the new measures Monday and said they would remain in effect for at least three weeks to help relieve pressure on the U.K.’s National Health Service.

He did not term them as a lockdown, but said people will only be allowed to leave home for a few “very limited purposes”.

NEW UNITED KINGDOM MEASURES EXPAND UPON PREVIOUS RESTRICTIONS

These include shopping for basic necessities “as infrequently as possible,” one form of exercise a day done alone or with household members, medical reasons or travelling to work that cannot be done from home or that is “absolutely necessary.”

“That’s all — these are the only reasons you should leave your home,” Johnson said in an address. “To ensure compliance with the government’s instruction to stay at home, we will immediately close all shops selling non-essential goods, including clothing and electronic stores and other premises, including libraries, playgrounds and outdoor gyms, and places of worship.”

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Social events, including weddings, baptisms and other ceremonies, will also be stopped. Funerals are exempt. Parks will also remain open for exercise, but any gatherings will be broken up.

As of Tuesday, the U.K. has 6,733 confirmed cases of coronavirus, with 336 deaths, according to statistics from Johns Hopkins University.

The Associated Press contributed to this report.