Sir Keir Starmer has blamed Boris Johnson for the group of antivax protesters who accosted him, saying the British prime minister knowingly pedalled “a conspiracy theory of violent fascists”.
In an interview with The Times, Mr Starmer said Mr Johnson had used a “deliberate slur without any basis in fact”.
“The PM knew exactly what he was doing. It is a conspiracy theory of violent fascists that has been doing the rounds for some time,” he said.
Mr Johnson last week accused his rival of failing to prosecute notorious paedophile Jimmy Savile while he was director of public prosecutions.
On Monday, police escorted the Labour leader away from demonstrators, some of whom accused him of “protecting paedophiles”.
“I have never been called a paedophile protector before. That happened yesterday for the first time in my life,” Mr Starmer said.
“If others want to argue that this is unconnected with precisely what the Prime Minister said one week before then let them make that case. But they’ll never persuade me that there is no link.
“It’s not about me, it’s the way we conduct our politics. I don’t want to see us go down the route that this potentially takes us.”
Earlier, Mr Starmer's spokesperson said he was “fine” after the incident.
A spokesman for the opposition leader told reporters: “He was back at his desk straight after. Police did their job.
“Obviously these things look sort of rougher on the camera often than they are.
“But, as I say, police did a good job, he was back at his desk afterwards, and he was off playing five-a-side football that night as normal.”
A host of Conservative and opposition MPs have called for Mr Johnson to apologise for what was branded a “scurrilous accusation” against Mr Starmer.
The prime minister’s official spokesman acknowledged his original words last week in the Commons were “capable of being misconstrued” and said that was why he subsequently issued a “clarification”.
“The Prime Minister clarified his remarks last week to make clear he was not suggesting Keir Starmer was individually responsible for the Savile decision,” the spokesman said.
He added: “I think the Prime Minister was making a political point about taking responsibility for organisations as a whole.”
Mr Johnson tweeted on Monday evening that the “behaviour directed” at the Labour leader was “absolutely disgraceful”.
Although Mr Starmer was head of the Crown Prosecution Service in 2009 when a decision was taken not to prosecute Savile, he had no personal involvement in the deliberations.
Labour said Monday’s incident occurred when Mr Starmer, who was accompanied by shadow foreign secretary David Lammy, was walking back from the UK Ministry of Defence after a briefing on the situation in Ukraine.
