The Conservatives are planning to demonstrate against London's congestion charge at every main entry point when the scheme commences on Monday.

An internal email leaked to the press revealed that the party intends to use the event to promote its mayoral candidate, who will be named on Sunday after a final decider between the former minister Steve Norris and Roger Evans.

Sent to activists in Camden, north London, the email says: "Conservatives will be taking part in demonstrations against the charge at all key entry points to the zone."

Confirming the ploy, Conservative spokesman on London Eric Pickles said: "This is just another tax that will hit the less well off in the capital the hardest.

"Conservatives are the only party to fight Livingstone's plans to introduce it. We will be campaigning on the streets and on the internet to oppose this unfair tax. "

Ken Livingstone admitted this week that Monday was likely to be a "bloody day".

The mayor called on drivers not to leave it until the day to pay, warning that the call centres might be jammed.

"There is a possibility that if people leave it to the last minute they might not get through," he said.

The scheme has received widespread national media attention as it is the first of kind to be tried in the country.

Ministers have refused to either condemn or condone the plans, saying that traffic charging schemes are now a matter for devolved bodies and local authorities.

Deputy mayor Nicky Gavron, who is Labour's official candidate for Livingstone's job in 2004, has given her backing and pledged to continue with the transport "experiment" if she wins the mayoralty.

However the Tories have sought to position themselves firmly on the side of the motorist, with all their mayoral candidates promising to scrap the £5 a day charge.

Norris, the Conservative candidate in 2000 who beat Labour to finish second in the race, remains favourite to win the nomination again.