Susan Hall accused Sadiq Khan of making an “outrageous comment” after he claimed she was “the most dangerous candidate I’ve fought against”.
The London Mayor made the remark as his Tory rival was grilled about past social media posts during a debate on LBC.
Ms Hall was asked about previous social media activity including liking a tweet branding Mr Khan a “traitor rat”.
She said: “I have apologised for my social media a long time ago and many times and I have learnt from that.
“I’ve learnt that you have to be very careful when you just flick on things without thinking twice about it.
“But I’ve apologised for that and I have learnt from my mistakes.”
Mr Khan said: “This is very serious because it affects me and my personal safety.”
Pressed on if he would feel safe in London if Ms Hall wins next month’s election, he went on: “I’m very careful what I say now and I’ve thought long and hard about this.
“I’ve fought three council elections, I’ve fought three parliamentary elections, this is my third mayoral election.
“The Tory candidate is the most dangerous candidate I’ve fought against.”
But the Conservative politician replied: “I think that’s an outrageous comment.
“And he would feel safer in a Susan Hall mayoralty because I will do things for the police force that will make Londoners safer.
“Look at your record on safety in London, it’s dreadful. This is the first time the Metropolitan Police has ever gone into special measures.”
It comes as Mr Khan is seeking a record third term in City Hall with Londoners taking to the polls on May 2.
