Prime Minister Gordon Brown has praised London Mayor Ken Livingstone for being an "inspirational" figure who has made a "huge difference" to the city.
Mr Brown commended Mr Livingstone on his record
The two Labour politicians, who have had widely reported differences in the past, visited the East End as part of Mr Livingstone's re-election campaign.
A recent opinion poll suggests the mayor is in a tough fight with his Tory challenger, Boris Johnson.
Up to 13 candidates are expected to be nominated for the 1 May election.
Visiting Canary Wharf, Mr Brown praised Mr Livingstone's policies on the environment, jobs, housing and transport.
He called him "an inspirational figure in London, a crusading mayor, and one that has made a huge difference".
In contrast he claimed Conservative candidate Boris Johnson planned to cut police and transport funding.
Boris Johnson has said he plans to fund an extra 440 Police Community Support Officers to patrol London's buses.
He also intends to free up more than 160,000 hours of police time by cutting back on bureaucracy, which he says will mean the equivalent of 78 more officers.
Alan Craig Christian Peoples Alliance & The Christian Party
Chris Prior Stop The Congestion Charge party
One London party
Gerard Batten UKIP
Ken Livingstone Labour party
Left List party
English Democrats party
Richard Barnbrook BNP
Sian Berry Green party
Winston McKenzie Independent
John Flunder Senior Citizens' Party
During his visit to Canary Wharf with Ken Livingstone, Gordon Brown spoke about schemes for young people in London, including training programmes, funding for post-16 education and apprenticeships.
He said: "Whenever I see talent wasted, potential unfulfilled or an ability that doesn't get the chance to prove itself, that's where Ken Livingstone and I and the government will be here to ensure there are opportunities for young people in the future."
Asked why he and the prime minister had teamed up after differences of opinion in previous years, Mr Livingstone said they were in agreement in opposing the Conservatives.
He added that "beyond all the flim-flam, trivia and personality politics, on the serious issues that decide whether people will work or get a home, they haven't changed".
Mr Brown and Mr Livingstone have disagreed on the financing of the Tube.
After Mr Livingstone left the Labour Party in 2000 to run as an independent mayoral candidate, Mr Brown said: "Some people might think Ken Livingstone is funny, but saddling London with him for four years is no laughing matter."
But he went on to win that election and was readmitted to the party before his second victory in 2004.
In recent weeks, Mr Brown has praised Mr Livingstone's record, but this is the first time he has hit the campaign trail on his behalf.
A YouGov survey for the Evening Standard, published earlier this week, suggests Mr Johnson has the support of 49% of London voters, with Mr Livingstone on 37% and Liberal Democrat candidate and former policeman Brian Paddick on 12%. Other candidates polled 3%.