Ken Livingstone, the hot favourite to win Thursday's London mayoral election, has appealed for people not to attend anti-capitalist protests in the capital on Bank Holiday Monday.
He said: "Whilst I'm sure a lot of the people going on Monday are planning to have a peaceful demonstration, it has become quite clear to police intelligence that there is a small core who will try and cause violence and people will get hurt in that."
He told BBC Radio 4's Today programme that because May Day was a traditional Labour celebration people should go to an event being held by the TUC at the Millennium Dome instead.
He said he believed in direct action, but added: "Violence is completely unacceptable in any civil society.
"We have struggled for hundreds of years to get a society based on the rule of law and the ballot box and we have that in place. There is no excuse for violence."
But shadow minister for London Bernard Jenkin said Mr Livingstone could not be trusted: "At any given time, Livingstone will say what he thinks people want to hear."
As the second day of the four-day protest got under way, Home Secretary Jack Straw promised a major operation to prevent violence: "It has to be policed effectively and it will be policed effectively."
Protesters are planning an eight-hour festival in Holloway Road, north London, in the Bank Holiday protest against global capitalism.
Last night hundreds of cyclists rode from Waterloo Bridge to central London in a peaceful protest at congestion and pollution on the roads.
Police fear there could be a repeat of last year's riots in the City of London on Monday when thousands of people are expected to descend on Parliament Square.
The demonstrators are expected to bring seeds and gardening equipment during the Guerilla Gardening action to symbolise the return of urban areas to nature.
Senior police officers fear the event could spiral into violence if hardcore activists use equipment such as spades and trowels as weapons.Reuse content