David Cameron was met with chants of "racist" at Prime Minister's Questions after he attacked Labour's mayoral candidate Sadiq Khan.
Mr Cameron accused Mr Khan of sharing a platform with extremists "again and again and again" as he waded into a row that has dominated the London mayoral race.
Zac Goldsmith, the Conservative candidate, has repeatedly claimed Khan has been guilty of "giving platforms, oxygen and even cover" to extremists.
In a rowdy House of Commons, the prime minister said:
He was interrupted by loud roars and shouts of "racist" from Labour backbenches.
Khan immediately responded, tweeting that he was "disappointed" the prime minister was using what he called Zac Goldsmith's "divisive, dog-whistling" tactics.
Labour MPs also condemned the remarks, with one MP accusing the prime minister of "demeaning his office".
Conservative MP Greg Hands leapt to Mr Cameron's defence, saying he was right to raise questions over Mr Khan's links.
Khan is currently the favourite to be chosen as Boris Johnson's successor when Londoners head to the polls on May 5.