Some commuters were told: 'we don't want to see you tomorrow' as police began a series of checks at rail and tube stations in the London suburbs.
A taskforce of 500 officers questioned passengers about their journey in a bid to cut the number of people on public transport.
The move came as Transport for London announced a third of its workers were off sick or self-isolating.
Staff shortages led to further cuts in the number of tube trains and a warning from mayor Sadiq Khan of even fewer in the days ahead.
Trains on the Amersham and Chesham branches of the Metropolitan Line were limited to one an hour in each direction and one every 15 minutes on the rest of the line.
Pictures posted on social media again showed overcrowded tube trains with passengers unable to comply with the 2 metre social distancing rule aimed at preventing the spread of the virus.
British Transport Police officers stopped commuters at ticket gatelines and told some to go home.
'We're in central London, we're on the tube, we're also out in the suburbs where people are starting their journeys. And we've had a few examples where we've persuaded people not to travel or indeed where we've not been that satisfied with their reason why they they're travelling today. We've made it clear that we don't expect to see them tomorrow.'
TfL will do everything possible to continue safely running a basic service for key workers, including our amazing NHS staff, but if the number of TfL staff off sick or self-isolating continues to rise - as we sadly expect it will - we will have no choice but to reduce services further.