London's bitter mayoral race pits Labour's Sadiq Khan against Conservative Zac Goldsmith: Margaret Evans

"Boris! Boris!"

The shouts ring out along the high street in Bexleyheath, one of London's outer-ring boroughs. They don't often get sightings of the city's most famous tow-haired politician, Boris Johnson.

He plows along like a lawnmower unleashed, zooming in and out of view and leaving a trail of political leaflets and the boom of his plummy accent behind.

There are shrieks of recognition, pats on the back and a whole lot of fumbling for cameras as people try to make the most of an unexpected selfie opportunity with London's outgoing mayor.

The man you don't notice is the one Boris is out trying to get elected. Zac Goldsmith, the MP and Conservative candidate to replace Johnson in London's top job, seems deeply uneasy with the informal walkabout shy, awkward.

He is a tall man, with a boyish face.But it quickly gets embarrassing as people crane their necks to look past him at Johnson, or Bojo, as he's called: the rock-star and the grey man.

But the seemingly mild-mannered Goldsmith is also the man accused of bringing the level of discourse in the mayor's race down to the most base levels.

He hasrepeatedly accused his main rival, the Labour candidate and front-runner, Sadiq Khan, of consorting with extremists.

"Those aren't allegations," he tells me just before the pained walkabout begins. "On many, many, many occasions he's shared platforms, given oxygen to, made excuses for people who have extreme views."

I tell Goldsmith I've just met a woman who says she's changed her mind and decided notto vote for Khan because she's just read on the internet that he has links to the Islamic State.

That has nothing to do with him, says Goldsmith.

Khan accused of closing down questions

"If you're standing for mayor of London, the most important city in the world with a big security remit, you cannot expect people not to scrutinize your extensive links to people who have extreme views," he says."Who wish to do the city harm.And it's absurd to think otherwise and it's irresponsible, I think, to try and close down those questions by talking about Islamophobia."

Before we can go too much further, Goldsmith's political handler teeters over on high heels to intervene, saying she thought we were only there to film "colour."

Goldsmith himself appears perfectly happy to continue the discussion. But apparently he can't be trusted, and he's whiskedaway.

Across town, Sadiq Khan's political handlers are also dealing with the media. They've arranged a photo-op to unveil Khan's new campaign billboards, promising more affordable housing.

Everyone stands around watching fourbig trucks emblazoned with said billboards inch forward and back and forward again until the bleary-eyed drivers get the thumbs-up from those in charge of the aesthetic.

Khan though, also an MP, is clearly deemed capable of being interviewed on his own. If elected,he would be London's first Muslim mayor.

He's on the record saying his work as a human rights lawyer has led him to be in contact with people whose views he doesn't share.He accuses Goldsmith's campaign of trying to paint him as an extremist, saying his Conservative adversary has brought "Trump politics" to the United Kingdom.

"Seeking to divide communities rather than bringing them together.What I want to do is be mayor for all Londoners, uniting our communities whether you are a Londoner of Jewish faith, Christian, Muslim, Hindu, Sikh, Buddhist, not a member of an organized faith, rich, poor, young, old, black, white, gay, lesbian.I'll be your mayor."

Labour dogged by allegations of anti-Semitism

That's a long list.Unfortunately for Khan it's a message that's been a lot harder to deliver, overshadowed as it has been, by a bitter internal row in the Labour party over allegations of anti-Semitism.

The party seems on the verge of imploding after the former London mayor and left-wing stalwart Ken Livingstone tried to defend a Labour MP accused of anti-Semitism by suggesting Hitler was a Zionist.

Livingstone and the MP in question have both been suspended from the Labour party, but the former mayor has refused to apologize for the remarks, and scenes of him being berated by a fellow Labour MP as a "Nazi apologist" have been splashed all over the television.

Khan was one of those calling for Livingstone's suspension.

"I've been quite clear that there's gotta [sic] be no place in life in 2016 for anti-Semitism.We've got to be sure we understand that racism is racism."

Fight could ruin opposition party's chances

Angry Labour loyalists say the fight could ruin the party's chances in the mayor's race, one of the few places they're expected to do well.Livingstone's supporters say the allegations are part of efforts to undermine the Labour leader Jeremy Corbyn's already tenuous hold on the party.

Political analyst Tony Travers of the London School of Economics says Corbyn's perceived failure to act quickly against those accused adds to the impression of a party in political meltdown.

"The fact that [the Labour leadership] haven't been able to handle it and convince anybody that they know how to handle it will inevitably wash back into the current London mayoral election and it's hard to see it helping the Labour candidate given that Labour have been accusing the Conservatives of using race-backed tactics in the race."

For one of the great, enlightened and cosmopolitan capitals of the world, it is an unseemly descent into crass politics, trolling the depths for religious and ethnic fault-lines.

If [Khan] were to win I think a lot of the British and London political class, whatever their politics, would see it as evidence that you know London really was sort of a cosmopolitan place.Outwardlooking. Comfortable with diversity.- Political analyst TonyTravers

It's even managed to overshadow Britain's obsession with class politics.Khan and Goldsmith represent two very different realities. Khan is the son of immigrants from Pakistan, a seamstress and a bus driver, raised on a council estate. Goldsmith is the son of a billionaire father who sent him to Eton until he was apparently expelled for smoking pot.

"They do physically represent sort of a class division in British society which it always worries about and people outside Britain also see," says Tony Travers.

It's not clear what impact, if any, recent developments will have on the outcome of the mayor's race. But some 44 per centof Londoners come from an ethnic minority background. And there are many in the city who say it's time for that to be reflected in the Mayor's office.

Travers says it would send an important signal.

"If [Khan] were to win I think a lot of the British and London political class, whatever their politics, would see it as evidence that you know London really was sort of a cosmopolitan place. Outward looking. Comfortable with diversity."

It doesn't feel like that just now.