Dozens of maskless passengers have been slapped with fines after compulsory face coverings were brought back in.
Face coverings became mandatory again on public transport from Tuesday amid rising concerns about the Omicron variant of Covid-19.
According to Transport for London, 152 people in the capital were ordered to pay a £200 fine on Tuesday.
Another 125 were asked to leave TfL services, and 127 were prevented from boarding a service or entering a TfL station.
TfL went on to say its staff had reminded more than 5,100 people to cover up their faces while travelling on its bus, Tube and rail network.
The Rail, Maritime and Transport union (RMT) previously raised concerns that train workers would be left to deal with angry passengers who do not want to wear face masks.
General secretary Mick Lynch said on Sunday that more resources were needed to police the use of face coverings.
Rules were reintroduced this week at 4am on Tuesday in a bid to fight the new variant of coronavirus.
Anyone who refuses to wear a mask will face a £200 fine the first time, but the charge will double for each offence.
This will make it £400 the second offence, £800 the third - up to £6,400.
But the rules will not apply to the hospitality sector in restaurants and pubs, at concerts or events - and Parliament cannot force MPs to wear them in the House of Commons.
Health secretary Sajid Javid defended the system which, as last year, will allow people to half their fine by paying within the first two weeks.
Speaking to Sky News he said: “Doing it in this proportionate way where it's for public transport, it's for retail outlets, I think is the right level of response on masks.
“It will be via government regulation and that means, I think, that people will take it seriously."
It comes after ten more cases of the Omicron variant were discovered in the UK bringing the tally across the country to 42.
A further seven cases of the variant were confirmed in England and another three in Scotland, the UK Health Security Agency said.
The B. 1. 1.529 variant, known as Omicron, was first identified in southern Africa, but has since been reported in at least 19 other countries but is expected to be far more widespread.
Cases in the UK have been found from Glasgow, to Liverpool, Norfolk and London as the government works to try and keep the new variant from spilling out of control.
