A Tory activist who wore a jacket with an offensive slogan about Tony Blair was told to take it off or face arrest.
Police are legally bound to act on a complaint about offensive words
Julia Gobert had been handing out fliers at Earl's Court Tube station, west London, on Tuesday morning, when she was approached by two officers.
She was told to remove it or face questioning after a complaint was made, said Tory councillor Barry Phelps.
Labour said it was in bad taste, Greens applauded the sentiment, not the words, and the Lib Dems have yet to comment.
Councillor Phelps, who was with Ms Gobert that morning, told the BBC News website : "One Labour gentleman complained to the station staff and so two British Transport Police came along and were clearly embarrassed.
"They said a member of the public complained and unless she desisted she will have to be taken in for questioning.
"I said to the officer: 'We are not in the business of making life difficult for you - Miss Gobert will take the top off'.
"It's not what I would have chosen to wear but there is no doubt that it captured the public mood."
A British Transport Police spokesman said: "People are of course free to wear what they like, but if a police officer receives a complaint about offensive language, they are bound to act upon the complaint."
The Public Order Act 1986 prevents the display of words or signs that could cause offence.