Dozens of stations on the London Underground network are closed, with the service operating at a reduced capacity following the outbreak of Covid-19 in Britain.
Up to 40 stations which do not interchange with other lines are closed, while the Waterloo and City line and Night Tube services will not run from tomorrow.
Transport for London said the plans were designed to allow critical workers to make essential journeys.
Buses in the capital will be reduced and people are being urged "not to use public transport for anything other than essential journeys".
London Mayor Sadiq Khan said: "People should not be travelling, by any means, unless they really, really have to. Londoners should be avoiding social interaction unless absolutely necessary, and that means they should be avoiding using the transport network unless absolutely necessary.
"London will get through these extraordinarily challenging times, and ensuring the capital's critical workers can move around the city will be crucial.
"Frontline staff across our health and care service - as well as those ensuring Londoners stay safe and can access food and other essentials - should be commended for their hard work.
"We owe it to them to do whatever we can to help them do their jobs effectively.
"I'm urging Londoners to only use public transport for essential journeys. Everyone should follow this and the other advice to help keep themselves and each other safe."
Transport for London said it would also be gradually reducing the frequency of services across the network from Monday, "to provide a service for critical workers to get to where they need to - ensuring that remaining services are not overcrowded".
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London Overground, TfL Rail, the DLR and London Trams will also be running fewer services from next week.
Britain's Transport Commissioner Mike Brown said: "The advice from Government is clear - people should now only be making journeys that are absolutely essential. We and our staff are doing everything we can to ensure that people who need to make essential journeys can continue to do so."
Passengers are being urged to check their journey on the Transport for London site before they travel.
The death toll from coronavirus in the UK has reached 144, the Department of Health said today.
The number of confirmed cases of Covid-19 was up by 643 to 3,269 according to the latest figures published earlier.
Around 80% of cases of Covid-19 will be a mild to moderate illness, close to 14% have severe disease and around 6% are critical.
Generally, you need to be 15 minutes or more in the vicinity of an infected person, within 1-2 metres, to be considered at-risk or a close contact.
The UK Ministry of Defence last night announced that 20,000 military personnel are being placed at "higher readiness" to help public services as part of a "COVID Support Force".
