Five hundred British Transport Police officers are being deployed to rail stations to "remind" people that only essential journeys should be made.
Sean O'Callaghan, assistant chief constable at BTP, said: "We are supporting rail operators and those key workers making their journeys home tonight by deploying 500 officers across the rail network nationally.
"They will be patrolling stations, supporting railway staff and reminding the public of the urgent need to follow the government advice - only those making essential journeys for work should be using the Tube and rail network."
He added: "We strongly urge the rest of the public to do the right thing and help us save lives by staying at home and slowing the spread of the virus."
Earlier on Tuesday, Health Secretary Matt Hancock called for London's Underground service to run "in full" during the coronavirus pandemic.
During his daily coronavirus update, Mr Hancock said running a full service would allow people to stick to social distancing measures and help NHS staff get to work.
"The best answer is that Transport for London should have the Tube running in full so people travelling on the Tube are spaced out and can be further apart, obeying the two-metre rule wherever possible," he said.
The health secretary added: "There is no good reason in the information that I have seen that the current levels of Tube provision should be as low as they are. We should have more Tube trains running."
His comments came after commuter trains were still packed on Monday morning, with pictures showing little space between commuters.
Last week, Boris Johnson urged the public to stop all non-essential journeys and keep a distance of two metres from each other to help slow the spread of the virus.
A partial shutdown of the London Underground began last week, with the Waterloo and City line completely shut, Night Tube services suspended and many stations closed.
Transport for London (TfL) further reduced the frequency of services from Monday, saying it would allow critical workers to "get where they need to".
The London Overground, TfL Rail, the DLR and London trams are also running a reduced service.
Some commuters have said that reductions in train carriages and timetables have caused overcrowding.
Nurse Paul Trevatt tweeted pictures of a busy platform at Finsbury Park station, saying he was "angry at the selfishness of other people".
Others reported that trains were more empty than usual.
Mayor of London Sadiq Khan's office said the health secretary's claims are "simply not true" and that the government must do more to stop people working unnecessarily.
A spokeswoman said: "The mayor has told ministers countless times over recent days that TfL simply cannot safely run a full service because of the levels of staff sickness and self-isolation.
"Nearly a third of staff are already absent - there aren't enough drivers and control staff to do it.
"The government must act urgently to get more people staying at home rather than going to work unnecessarily - that means taking the difficult decisions they are refusing to take to ban non-essential construction work and provide proper financial support to freelancers, the self-employed and those on zero-hours contracts to stay at home."
According to the Department for Transport, demand for rail travel across the UK has fallen by up to 69% on some routes.
In a televised address to the nation last night, the prime minister urged people to stay at home unless they had good reason.
Boris Johnson said people should only go out for essential supplies, exercise once a day, medical needs, to help vulnerable people or to get to work if "absolutely necessary".
Failure to follow the new rules, which will be in place for at least three weeks, could see police officers dispersing gatherings and imposing fines.
Those flouting the rules could be fined anything from £30 upwards.