The proposed extension could come into force by February 2007
Up to 100 vehicles are taking part in a go-slow protest over plans to expand the congestion charge into west London.
Protesters gathered at Addison Road in west Kensington at 1030 BST before driving into central London.
Their route takes them via Parliament Square, Embankment, the Strand, Trafalgar Square, Piccadilly Circus and Regent Street.
Gordon Taylor, chairman of the West London Residents' Association, said there is anger over the extension.
Campaigners said the extension is unfair, saying only 5% of the roads covered in the extended zone suffer congestion.
They said drivers should pay for the use of congested roads only, rather than the camera system used in the existing congestion charge zone in central London.
Mayor Ken Livingstone last year gave his backing to the zone's extension, which would take it into the borough of Kensington & Chelsea.
Businesses there fear falling trade if the charge comes into force.
If it is passed, the extension could be in place by February 2007 at the earliest, Transport for London (TfL) said.
A TfL spokesman said any public concerns about the extension would be taken into account.
"In the coming weeks there will be the start of full public consultation of the proposed western extension," he said.
"Everyone will get their opportunity to put any concerns forward to TfL and the mayor about the proposals."
A spokesman for the National Alliance Against Tolls said: "The planned extension is not the end. The long term aim of the Mayor and TfL is tolls on most London roads.
"Tolls just displace traffic on longer routes down less suitable roads and can harm businesses in the charging zones. The way to stop this threat is to scrap the charge completely."