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."
@TfL @SadiqKhan @BorisJohnson you lot claim to care about the public I just watched and 86 bus at Stratford bus station pull in and not one person Is social distancing because there’s to many people on buses either put out more buses @ peak times or just go back to lockdown pic.twitter.com/eylN6q4TqV— ᒍᗩY_ᗷITᔕ_ Risky Roadz X BOTBOT x Grime Gran (@Jay_Bits_) May 13, 2020
Transport for London (TfL) said 5,674 more passengers travelled on the Tube on Wednesday compared to the previous day - a 7% increase.
Passengers journeys are still 93% lower than this time last year.
TfL ran 70% of its Tube services during the morning and 82% of its usual bus services.
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,
A TfL spokesman said: "We are doing everything possible to return Tube and bus services to normal levels under extraordinarily difficult circumstances, with many staff still off sick, shielding or self-isolating.
"In keeping with the government's plans for the national rail network, next week we aim to increase to 75% of Tube services, 85% of bus services, restore the Circle Line and reopen some closed Tube stations."
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.