Sir Keir faced baseless allegations of “protecting paedophiles” and chants about the sex offender from protesters before being bundled into a police car for protection.
Former Cabinet minister Julian Smith said the Prime Minister must withdraw the Savile slur for the sake of Sir Keir’s security in the wake of the incident on Monday.
Mr Johnson tweeted that the “behaviour directed” at the Labour leader was “absolutely disgraceful” but did not address the nature of the abuse.
Officers stepped in to protect the Opposition leader as the group, some protesting against Covid measures, followed him from outside Scotland Yard.
On at least two videos posted to social media, a man and a woman were heard shouting about Savile to the Labour leader, as he walked with shadow foreign secretary David Lammy.
At least one person was heard repeatedly shouting about “protecting paedophiles”.
Last week, an under-pressure Mr Johnson accused Sir Keir of having “used his time prosecuting journalists and failing to prosecute Jimmy Savile” while director of public prosecutions (DPP).
After the incident, Mr Lammy said it was “no surprise the conspiracy theorist thugs who harassed” Sir Keir and himself “repeated” Mr Johnson’s slurs.
Nicola Sturgeon, said on Twitter: “Any politician choosing to weaponise online conspiracy theories against opponents knows fine well that they risk stirring up the kind of hate and abuse that Keir Starmer experienced today.
“If he has any decency at all, the PM will now apologise unreservedly.”
Mr Smith, who previously served as Mr Johnson’s Northern Ireland secretary, tweeted: “What happened to Keir Starmer tonight outside parliament is appalling.
“It is really important for our democracy & for his security that the false Savile slurs made against him are withdrawn in full.”
Chris Bryant, the Labour MP who chairs the Commons Committee on Standards, said: “This is appalling. People were shouting all sorts at Keir, including ‘Jimmy Savile’.
“This is what happens when a Prime Minister descends into the gutter and recycles lies from hard-right conspiracy theorists. Political poison has an effect. Johnson has no moral compass.”
Labour’s Dame Angela Eagle tweeted: “Proud of yourself stoking up the worst Internet conspiracy theories PM? Disgusting new low.
“PM must now personally & unconditionally apologise for his disgusting lie on the floor of the House of Commons where he first made it.”
Scotland Yard said two arrests were made following clashes between police and protesters on the Victoria Embankment after Sir Keir was taken to safety.
A Metropolitan Police statement said: “Shortly after 5.10pm on Monday, February 7, a man who had been surrounded by a group of protesters near to New Scotland Yard, was taken away from the scene by a police car.
“A man and a woman were arrested at the scene for assault of an emergency worker after a traffic cone was thrown at a police officer.”
Home Secretary Priti Patel said the scenes were “completely unacceptable” and that she was in contact with police over the incident.
Mr Johnson tweeted: “The behaviour directed at the Leader of the Opposition tonight is absolutely disgraceful. All forms of harassment of our elected representatives are completely unacceptable.”
Footage posted to social media showed Piers Corbyn, the Covid-19 conspiracy theorist brother of former Labour leader Jeremy, addressing the crowd before the incident and later leading chants of “resist, defy, do not comply”.
Sir Keir, surrounded by police, was followed down the street while being targeted with shouts of “why aren’t you opposing?” and “traitor”.
Someone yelled: “You should be hung.”
“Why did you go after Julian Assange, why did you go after journalists?” one man shouted.
It was understood Sir Keir was not harmed during the incident.
After he was taken to safety, an officer was called a “pathetic little thug” during angry exchanges.
Protesters were seen displaying signs opposing mandatory vaccination and the use of restrictions to prevent Covid-19 deaths.
Mr Johnson first shared the falsehood in the Commons on January 31.
Under vast pressure, he later clarified his remarks, insisting he had not been referring to Sir Keir’s “personal record”.
But he did not apologise for the “scurrilous” smear, which long-standing adviser Munira Mirza cited as the reason for resigning as No 10’s head of policy.
Sir Keir apologised while director of public prosecutions in 2013 for the CPS having failed to bring Savile to justice four years earlier.
But there is no evidence that Sir Keir had any personal role in the failure to prosecute the man who was one of Britain’s most egregious sex offenders before his death in 2011.