Image copyright AFP Image caption The Conservative and Labour candidates' attacks on each other became even more direct in a recent debate

Elections are rarely chummy affairs - but over the past few days relations between the two leading candidates in London's mayoral race have become distinctly unfriendly.

Conservative candidate Zac Goldsmith has been accused of running a "desperate" and "racist" smear campaign against his Labour opponent Sadiq Khan.

Clearly incensed at the accusation - which he rejects - Mr Goldsmith has pushed full steam ahead with his claims that the Labour candidate has given "oxygen to extremists".

Mr Khan, in turn, fiercely denies this.

None of the above sounds particularly positive. Ask either of these candidates who is being negative, though, and they will tell you it's the other guy.

Image caption Conservative supporters showed up at a recent Labour campaign event

If a negative election campaign is one that seeks to discredit one's opponent, then both sides have been doing that to varying degrees.

Let's start with the language used by the Conservatives: "radical", "divisive" and a "dangerous experiment". That's how Mr Goldsmith has described his opponent.

Labour argues this was to plant the false idea in the minds of voters of their man as a radicalised Islamist.

The Tories say that Labour has deliberately misinterpreted the word, and they were referring to what they see as Labour's left-wing, radical direction under new leader Jeremy Corbyn.

The Conservatives have put out several posters to that effect, presenting Mr Khan as "Corbyn's man", which have popped up at Labour events.

Image caption A Labour poster poking fun at Zac Goldsmith that looks remarkably similar to...

Image copyright Conservative Party Image caption ...a poster by the Conservatives poking fun at Sadiq Khan

Just this week Conservative activists were seen waving anti-Khan banners at a Labour campaign visit in Sutton, south-west London. Mr Goldsmith's team would not confirm whether the gatecrashers were part of their campaign.

But Labour has been getting in on the negative poster adverts too, such as the one on housing that bore an uncanny resemblance to Conservative posters.

Mr Khan too has called his rival "divisive" and drew up a mock CV on social media at the start of the year painting the Conservative as "a serial underachiever" who wasn't up to the job of mayor.

Recently, the criticism has become even more direct.

At a debate on Tuesday Mr Khan began by saying he was "disappointed" with the strategy used by his opponent - and Mr Goldsmith fought back, telling him: "I have never referred to you by your religion."

Neither wants to be presented to potential voters as an Islamophobe or indeed as an Islamist sympathiser, but winners often take no prisoners.

Find out more about who else is standing in the London elections.