London mayor election: Candidates, issues and powers
It's the fourth time Londoners have been asked to elect a mayor for western Europe's biggest city.
Two people have done the job since it was created in 2000. The same two big personalities are locking horns this time, in a campaign which has veered between foul-tempered outbursts and celebrity knockabout.
Many in the capital have been gripped by the peculiar contest that is the "Boris vs Ken show". They are just about the only politicians familiar and idiosyncratic enough to be popularly known by their first names alone.
Both have made a splash on TV comedy panel shows in the past. Both have at times landed themselves in hot water with their outspoken remarks. Both are looked on by their parties with a strange mixture of fascination and trepidation.
Boris Johnson is the colourful Conservative. A classical scholar, he has endeared himself to many with his bumbling charm and mop of uncontrollable blonde hair. His gaffe-prone past sometimes distracts from a considerable intellect.
Mr Johnson is almost unique in his ability to reach well beyond traditional conservative support.
Ken Livingstone, known as "Red Ken", has experience of running London going back to 1981. He was the leader of the Greater London Council (later abolished), and was a vocal opponent of Margaret Thatcher.
When he first sought the nomination to be Labour's mayoral candidate in 1999, he was blocked by Prime Minister Tony Blair. But Mr Livingstone turned that to his advantage, and successfully stood as an independent candidate the following year. He is now back in the Labour fold.
This year's election exploded into life with public and private rows about tax. Mr Livingstone had paid income from broadcasting into his personal company. His opponents said it was a way to avoid paying full income tax - something Mr Livingstone denied.
The Labour candidate said his tax arrangements were "exactly the same" as those of Mr Johnson, who had also briefly been a director of a media company.
At this point Mr Johnson went apoplectic - and repeatedly called Mr Livingstone "a liar". The language got even more colourful during a heated encounter in a lift after the first mayoral debate.
The lift incident and accompanying fall-out has left the others in this race - Brian Paddick for the Liberal Democrats, Jenny Jones for the Greens and the independent Siobhan Benita - rather annoyed and bemused.
All the candidates now say they want to get back to discussing the real issues for London in this contest: transport, crime and the economy. But local government experts say there are significant constraints on the ability of whoever is mayor to implement their ideas.
Mayor of London is a very different job to, say, mayor of New York. In the US, mayors often have direct control over a much greater range of services. They also have more extensive powers to raise finance - by issuing bonds or raising taxes.
By comparison, the mayor of London must make do with a slice of the council tax collected by the patchwork of local authorities (called boroughs) which supply most of the services to Londoners.
The mayor has important responsibilities for the strategic direction of transport, policing and economic development in the capital, but not direct day-to-day control over police operations or the running of the trains and buses.
In terms of direct powers, the position of London mayor is closer to, say the mayor of Paris than his US counterparts. But the number of people voting in the London election is much larger than in the French capital, which gives the job added profile.
Most of the really important public services - like education and health - are, however, still largely controlled by the UK government. The mayor may be able to argue the case that London should get extra funding, but otherwise their influence has its limits.
All the candidates in this election say they will be able to help with a whole range of problems - from the price of heating fuel to the costs of childcare. Their ability to achieve success in these disparate areas will depend on how effective they are as individuals.
A charismatic mayor with a direct mandate from London's voters could have a much greater impact than the constitutional limits of the job might suggest.
But that's true for any leader.