As nominations open for candidates seeking to become the next mayor of London, Ken Livingstone has warned that "the stakes are very high".

The Labour candidate and current mayor used his campaign launch on Tuesday to warn voters that Londoners face a stark choice in the May 1 elections between himself and the Conservatives' Boris Johnson.

Setting out the key areas where he would "keep moving our city forward", Livingstone pledged to improve transport, housing, crime and community relations in the capital.

"If you want to take London on the path that continues to move forward, you have to vote for it. Or you can choose to rewind on a path that simply does not exist any more," he said.

The mayor said he would continue the £16bn Crossrail project and "transform one of our city's biggest transport assets" through the £1bn modernisation programme of the Tube.

On housing, he said he would ensure that "half of new housing in London must be affordable to rent and buy".

"Boris Johnson wants to abolish that 50 per cent policy," he said. "It would block London families from having real opportunities to get affordable housing and price housing even more out of the hands of ordinary Londoners."

Increased police numbers had led to a reduction in crime, said the mayor, and he pointed to Johnson's calls for "big ticket savings in the police budget".

"Cutting crime for five years in a row needed serious decisions. Londoners wanted it and I have delivered it," he argued.

Policy pledges included an extra 1,000 police over the next year and extending the provisions of the Freedom Pass for older and disabled people.

Livingstone also wants to introduce a £25 per day charge for high emitting vehicles entering the congestion zone and invest in a £78m programme to set up youth centres.

He added: "These new commitments to Londoners, building on what has been achieved together, will help keep London successful, ensure that Londoners share in that success, and help protect London's environment and quality of life."

Meanwhile Henley MP Johnson is holding a series of engagements focused on crime issues and also addresses an event hosted by Wales in London, an organisation that champions Welsh interests in London.

As the campaign formally begins, Johnson has a 12-point lead over Livingstone, according to the latest opinion poll.

Monday's YouGov poll put Johnson on 49 per cent, Livingstone on 37 per cent and Liberal Democrat candidate Brian Paddick on 12 per cent.