Steve Norris and Ken Livingstone both say the mayoral race is too close to call, just hours before a result is declared.
Candidates for the Mayor of London and London Assembly will gather at City Hall in Southwark tonight for the official declaration of results.
Polls have indicated a tight race between the Mr Norris, Conservative, and Mr Livingstone, Labour.
Liberal Democrat Simon Hughes will be hoping his two major rivals suffered from protest voting.
Some voters may have disapproved of Mr Livingstone returning to Labour, while Mr Norris was criticised for his role with the rail infrastructure company Jarvis.
Two separate polls yesterday found Mr Livingstone heading off Mr Norris by about 7 per cent.
Under the preferential voting system, voters cast two votes for their first and second choice of Mayor.
If no candidate wins more than 50 per cent of first percentage votes, the two highest-polling candidates go into a run-off, with their first and second preference votes added together to find the winner.
Members of the London Assembly are elected from constituency and city-wide lists.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules hereComments are closed on this article