Cabbies are the latest malcontents to fall foul of a new law banning unauthorised protests near Westminster.
Cabbies are angry about proposed congestion charge changes
They had planned to drive around Parliament Square, in protest at proposed changes to laws governing cabs and the congestion charge.
But police have warned them they could face arrest, so now London Cab Drivers Club is warning members not to show.
The new law was introduced in August, banning protests around Westminster which do not have police permission.
Demonstrators must seek police consent for any protest within half a mile of Westminster under the Serious and Organised Crime Act.
The cabbies are annoyed about aspects of the Transport for London Bill 2005, which includes changes to Hackney Carriage laws and to the scope of the congestion charge.
But they had not realised that by driving around Parliament Square four times in protest at 1300 GMT on Thursday, without alerting police, they were breaking the law.
Alan Fleming, chairman of the London Cab Drivers' Club, said police had warned him that "any driver that goes there runs the risk of being arrested and a very big fine".
"And also the organisers [could be fined] - it came as quite a bit of a shock," he added.
He told the BBC News website they planned to organise another demonstration, with police permission, in January when the Bill gets its first reading.
In a statement, Charing Cross Police said: "Should the organisation wish to hold a demonstration or procession police will work them with to facilitate this given appropriate notice."
The new law was initially intended to remove Brian Haw, an anti-war protester who has camped in Parliament Square for four years.
But Mr Haw successfully fought off the latest attempt to evict him in the High Court, by arguing his protest pre-dated the legislation.