London Underground passengers have been crowding on to Tube trains, despite warnings to limit non-essential travel.
Images from Monday's rush hour show busy carriages, which unions say left staff feeling "furious" as it rendered social distancing "impossible".
London Mayor Sadiq Khan has urged workers to stay at home and said public transport should only be used by key workers, otherwise "people will die".
Some passengers have said a reduced service means trains are busier.
Transport Secretary Gant Shapps said plans were in place to ensure key workers travelling on trains had "space to be safe".
Mr Shapps tweeted: "Concerning to see images of packed trains this a.m. The advice is clear: Stay home if possible. That is the way to save lives.
"We are working with train operators to introduce a small number of trains for key workers to have space to be safe."
Aslef union's district organiser Finn Brennan tweeted: "Still heavy loading on some Tube lines this morning making social distancing impossible.
"This is endangering the health of the vital workers who have to use the system."
He called on the government to act, adding: "I'm being sent pictures of crush loaded platforms at some Jubilee line platforms this morning.
"Drivers and other frontline staff are furious."
Transport for London (TfL) said there had been a 70% fall in the number of passengers on the Tube network during the week and a 40% fall in bus use across its network.
There was also an 87% fall in Tube passengers at the weekend compared to the same time last year.
However, some commuters have complained trains have become busier.
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 the past week.
She said she felt "more concerned" travelling because fewer services meant there were "more people in a confined space".
Mr Khan said: "Londoners should not be travelling by any mode of transport unless it is absolutely necessary, and only critical workers should be using public transport."
He added: "TfL will continue to do everything it can to provide a safe service, but like many organisations it is dealing with rising absence levels and needs Londoners co-operation in these challenging times."
Mick Cash, general secretary of the Rail, Maritime and Transport union, said: "There is still enormous personal pressure on the Tube workforce who are exposed to levels of social contact that the government say are unacceptable for the wider public.
"The only people using our transport services should be essential workers who have to travel."
Last week 40 Tube stations were closed for the foreseeable future in an attempt to reduce the spread of the coronavirus outbreak.
There is no night Tube and bus services have also been reduced.
The Waterloo and City line is shut and TfL said it would gradually reduce other parts of its network.
The @metline train is now nearly full..cannot maintain 2m distance and there are people coughing. How does this make any sense?— Deepa Santhosh (@deepasanthosh) March 23, 2020
These include the London Overground, TfL Rail, the DLR and the tram network in south London.
Prime Minister Boris Johnson has said the spread of the Covid-19 outbreak is much faster in the capital compared to the rest of the country.
As of 22 March, London had 1,965 confirmed cases, representing 39% of 5,018 confirmed cases in the UK.