Sadiq Khan has admitted his "regret" at giving the impression he shared the views of extremists.
At a stormy BBC debate with four of his fellow candidates for London mayor, the Labour front-runner and the Conservatives' Zac Goldsmith clashed again over claims the Tooting MP had "given platforms and oxygen" to extremists.
Mr Khan said he had "never hidden" the fact that, as a former chairman of Liberty and a human rights lawyer, he had acted for "some pretty unsavoury characters".
And when asked if he regretted sharing a platform with extremists, he said: "I regret giving the impression I subscribed to their views and I've been quite clear I find their views abhorrent."
Mr Goldsmith, MP for Richmond Park and North Kingston, also hit back at questions he or his campaign team had labelled his Labour rival an "extremist" himself.
He said: "My campaign has been overwhelmingly positive.
"I have made it very, very clear that I have never suggested that Sadiq Khan was an extremist in any way at all.
"The point I have made, and Londoners have made and the newspapers have made on a regular basis is that Sadiq Khan has given platforms and oxygen and even cover to people who are extremist and I think that is dangerous."
He added the claims made against Mr Khan himself had been down to "only a few nut jobs on Twitter".
Mr Khan also said he was "not reassured" that the capital's emergency services were prepared to respond to a Mumbai-style terrorist attack.
He added he intended to review the police, fire service and security services if he was voted into office next month.
He said: "If there was a Mumbai-style attack in London, are there sufficient armed response units at the moment?
"I want reassurance that we are ready. If you've closed down ten fire stations, there are 30 more fire engines being lost from London, half of London’s firefighters live outside London - I worry about London’s security.
"I need to be reassured as the mayor of London that we are all going to be safe.
"I'm not reassured yet - I want to be reassured."
The five candidates, also including Sian Berry (Green), Caroline Pidgeon (Liberal Democrat) and Peter Whittle (UKIP), faced a grilling from an audience of around 130 Londoners on the BBC London show, hosted by Daily Politics presenter Andrew Neil.
The candidates set out their positions on security, housing and transport - resulting in fractious exchanges at times.
Mr Goldsmith said he would retain Metropolitan Police numbers at 32,000 and put an additional 500 officers on the Tube, while Ms Berry said she was "scared" about the terror threat but did not believe increasing the numbers of armed officers was the solution.
In response to a question about community cohesion and Islamophobia, Mr Whittle drew applause when he said he favoured a partial ban on full-face veils in certain public places.
Mrs Pidgeon said she was "fed up with this mud-slinging" during the campaign and also pledged to increase the presence of police officers on the London transport network.
Later on, when Mr Khan heckled Mr Goldsmith over a plan for £450,000 starter homes, the Tory candidate hit back saying: "We've had this argument - don't repeat it because you are on television."
In response to a question about London's housing crisis, Mrs Pidgeon pledged to build 50,000 council homes, while Ms Berry said she would demand London gained powers to control the rental market.
Speaking afterwards, audience member Nasrin Master, 41, said she felt she did not get a straight answer to her question about defining a Londoner in relation to Mr Khan's planned "first dibs" policy on newly-built properties.
The magistrate added: "I thought that Zac Goldsmith came across a lot more smooth and he seemed a lot more calm in his approach, whereas Sadiq Khan seemed slightly on edge, which makes me wonder how he would handle really serious controversial issues that he is likely to face if he was mayor."
She said Mr Khan's need for reassurance in the security services was "odd" and he could "alienate" the emergency services.
The BBC said the candidates appearing on the show had been chosen based on their levels of support at recent London elections.
The remaining seven candidates were offered the chance to pre-record clips to air at the start of the 45-minute programme, which aired on BBC One London.