Commuters in London said social distancing was “next to impossible” as many made their first journeys to work since lockdown rules were eased.
People in England are being encouraged to return to work if they cannot work from home.
The government said it would have to “take steps” if too many people used public transport.
One commuter said most people were not wearing masks, leaving him fearing “a second wave of infection”.
Passengers using public transport should stay 2m (6ft) apart and wear face coverings, under government guidelines.
“It is next to impossible to social distance on the Tube,” Matt Hickson told the BBC.
The 47-year-old street works inspector said he saw “less than 10% of commuters wearing masks” on his London Underground journey.
“People are taking liberties not only with their own health but with other people’s.
“There could be a second wave of infections coming.”
Transport for London (TfL) said passenger numbers between 05.00 and 06.00 BST were up 9% compared to the same time last week.
A passenger was taken ill on the Victoria line at about 07.20 leading to delays and overcrowded trains, TfL said.
The train was held while the passenger received medical attention. The incident is not thought to be coronavirus related.
“Any notion of self-distancing on buses or tubes is going to be impossible,” said Drew Aspland.
The 36-year-old walked to work in central London from Bethnal Green after seeing his bus “back to pre-lockdown levels of passengers”.
Trains at London Waterloo have been running at 45% of normal capacity since Monday, after reducing services by 75% since the earlier stages of the lockdown.
Services are expected to rise to 82% from next Monday.
Several of London’s main roads experienced higher traffic than in recent weeks.
Queues of up to 45 minutes were recorded on a five mile stretch along the East India Dock Road.