Up to 40 stations on the London Underground network are to be shut as the city attempts to reduce the spread of the coronavirus outbreak.
Transport for London (TfL) said there would be a partial shutdown of the network from Thursday morning.
There will be no night Tube and bus services will also be reduced, it said.
Mayor of London Sadiq Khan said people should not be travelling and warned services were "likely to reduce, potentially very significantly".
The move comes after Boris Johnson said the virus was spreading faster in London than other parts of the UK.
Latest government figures show there have been more than 900 confirmed cases of coronavirus in London and 34 people have died in the city.
Earlier this week, the prime minister urged people to work from home and to avoid bars, pubs and restaurants.
Schools will close from Friday.
Thurday 8:30am on London Underground, Embankment stop - definitely most Londoners already working from home 💪👏 pic.twitter.com/22m4lwRiEl— Michael Panagopoulos (@Mikepanagop) March 19, 2020
There has been a large drop in the number of passengers on the London Underground network during the week.
However, some commuters have complained that trains have become busier, particularly where fewer services were running on lines.
One key worker, a nurse who asked to remain anonymous, said it was "a lot busier" during her journey on the District Line compared to earlier in the week.
She said she felt "more concerned" travelling because fewer services meant there are "more people in a confined space".
"It didn't feel like people were staying at home," she said.
Tube in London this morning - it’s like there’s no pandemic. I’m sure not everyone here is a key worker. Companies want their staff to come in and staff obviously don’t want to lose their jobs or money. pic.twitter.com/1Nr5lbdOmc— leanahosea (@leanahosea) March 19, 2020
Maria, a commuter who was getting the Tube from Oxford Circus after finishing her cleaning shift, told the BBC she was "worried" about the situation but would only stop working when told to do so by the firm that employs her.
"I have bills to pay and if I don't work I don't get paid," she said.
'Follow expert advice'
Nine stations are currently closed but commuters have been advised to check the TfL website in case more are shut.
From Friday, the Waterloo and City line will shut completely and from Monday, TfL said it would gradually reduce other parts of its network.
These include the London Overground, TfL Rail, the DLR and the tram network in south London.
Transport bosses have said staff who are available to work will be redeployed "to ensure the resilience of the regular Tube and Overground services".
From Monday, buses will run on a Saturday timetable, although night services will continue "to provide critical workers with a reliable night option", TfL said.
Speaking at Mayor's Question Time, Mr Khan said the number of services would probably "continue to reduce, potentially very significantly, over the days and weeks ahead", but TfL would "make sure essential workers can still get around".
He criticised Londoners who were not following official guidance and travelling around the city.
"I can't say this clearly enough: people should not be travelling by any means unless they absolutely must.
"I want to see more Londoners following the expert advice, which means it's critical that we see far fewer Londoners using our transport network than is currently the case," he said.
While the centre of the capital is quieter than it would be on a regular Thursday, there is still some activity on the streets.
Tourist spots like Trafalgar Square are quiet with only the odd person taking selfies, but roads through the city remain busy and there are people in traditionally busy places like Oxford Street.
Underground stations facing closure from Thursday:
- Bakerloo Line: Lambeth North, Regents Park, Warwick Avenue, Kilburn Park, Charing Cross
- Central Line: Holland Park, Queensway, Lancaster Gate, Chancery Lane, Redbridge
- Circle Line: Bayswater, Great Portland Street, Barbican
- District Line: Bow Road, Stepney Green, Mansion House, Temple, St James's Park, Gloucester Road
- Jubilee Line: Swiss Cottage, St John's Wood, Bermondsey, Southwark
- Northern Line: Tufnell Park, Chalk Farm, Mornington Crescent, Goodge Street, Borough, Clapham South, Tooting Bec, South Wimbledon, Hampstead
- Piccadilly Line: Caledonian Road, Arsenal, Covent Garden, Hyde Park Corner, Bounds Green, Manor House
- Victoria Line: Pimlico, Blackhorse Road
TfL advised passengers to check the website for live updates.
Although there are no plans to suspend the congestion charge, a spokesperson for TfL said: "Some NHS staff are already eligible for reimbursements from the congestion charge in certain circumstances.
"Patients clinically assessed as too sick to travel by public transport are eligible for reimbursements from both the congestion charge and the ULEZ."
Tom Edwards, BBC London transport correspondent
We all knew it was coming but it was still a shock and it feels like this is just the beginning.
To deal with staff absences, TfL is cutting its services. Initially 40 quieter stations will close and services will be reduced - but a crucial phrase is that it "may reduce further".
TfL wants to protect a service for "critical workers", in particular, hospital staff.
And the language has changed totally - from just a few days ago where public transport was "safe" the mayor now says people should avoid using transport unless "absolutely necessary".
In private, train companies say it's inevitable they will also have to reduce services.