London Mayor Boris Johnson has been filmed approaching motorcyclist protesters in central London in behaviour they say is "threatening".
Members of the group No to the Bike Parking Tax were protesting in Trafalgar Square on Wednesday evening.
Mr Johnson was caught on film asking a protester: "Do you want to stay in the bus lanes or what? Then stop this protest."
The group said his behaviour was "disgraceful, bordering on blackmail".
Warren Djanogly, chairman of the group No to the Bike Parking Tax, said: "This would seem like a thinly veiled threat, bordering on blackmail."
A Mayor of London spokesman responded saying that by blocking off roads around Trafalgar Square as part of their protest, the demonstrators were "inconveniencing thousands of Londoners by jamming up a large part of central London".
Mr Djanogly demanded an apology from the mayor.
"We feel an apology should be given. He threatened quite a few people. This is utterly disgraceful."
"We were exercising our fully legal right to protest."
Protesters were campaigning to have all parking taxes for motorcycles and scooters in Westminster abolished.
This is a separate issue to the motorcycles in bus lanes trial currently taking place.
The 18-month Transport for London (TfL) trial allowing motorcycles, mopeds, scooters and tricycles - but not those with sidecars - to travel in most red route bus lanes ends on 5 July.
TfL will make recommendations to the mayor, who will make the final decision as to whether the scheme becomes permanent.
A Mayor of London spokesman said: "The mayor promised motorcyclists access to bus lanes on a trial basis, but made it clear they would need to develop a mutual respect with others using the lanes for it to work well.
"He realises their desire to protest about parking charges in Westminster, but was making the point that inconveniencing thousands of Londoners by jamming up a large part of central London only damages the respect that they need for the bus lanes trial to be successful."
Dr Leith Penny, Westminster council's strategic director of city management, said: "With the huge demand for on-street space in Westminster, we think it's only reasonable that motorcyclists should pay a modest sum of £1 a day to park."
He added that the council "does not shy away from robust debate".