Labour candidate for London Mayor Sadiq Khan said he is "not reassured" the capital's emergency services are prepared to respond to a Mumbai-style terrorist attack.
The front-runner said he intends to review the police, fire service and security services if he is voted into office next month.
Speaking at a stormy BBC debate with four of his fellow candidates, 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."
Mr Khan and the Conservatives' Zac Goldsmith clashed again over claims the Tooting MP had "given oxygen" to extremists.
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.