Conservative candidate Boris Johnson was leading Labour's Ken Livingstone in the race to become London mayor tonight with nearly three quarters of the votes counted.
Mr Johnson led incumbent mayor Livingstone in eight of the capital's 14 mayoral constituencies, and by a wide margin in three of them.
Liberal Democrat candidate Brian Paddick was trailing at a distant third.
"At this moment it looks as if Boris Johnson is ahead," said London Minister Tessa Jowell, chief of Labour's campaign in the capital, cautioning that it was too early to declare a winner.
She suggested Mr Livingstone's support had been hit by a wider national dissatisfaction with the government, which sank to its worst performance on record in local elections held elsewhere in England and Wales yesterday.
"If you get a move against the government as we've seen across the country, London is not immune," she said.
But the outcome of yesterday's poll remained uncertain as live count figures displayed at the mayor's City Hall on the River Thames opposite the Tower of London only showed voters' first preference choices.
The final result will not be known until second preference votes from the other unsuccessful candidates are added to the two leading contenders' totals - unless one of them passes the 50 per cent mark on the first count.
The live data from three counting centres tallying around 2.4 million votes showed Mr Livingstone performing well in inner London areas, his long-established base of support.
Mr Johnson was faring best in the capital's traditionally Conservative-supporting suburbs, with four times as many votes as Mr Livingstone counted so far in the south east constituency of Bexley and Bromley.
The counting started at 8.30am and the final declaration is not expected before 8.30pm and could be much later.
The winner will be in charge of an £11 billion budget covering London's public transport, police and fire services.
He will also oversee preparations for the 2012 Olympics, and be responsible for promoting capital-wide policies on housing, the environment and its economy.