Britain's Boris Johnson said Sunday the security services were stepping up monitoring of convicted terrorists released early from prison, as the London Bridge attack became embroiled in the election campaign.

The prime minister revealed around 74 people with terrorist convictions had been released early from prison in a similar way to Usman Kan, who left jail last December and went on to stab two people to death in Friday's rampage.

"They are being properly invigilated to make sure there is no threat," Johnson told the BBC in an interview.

"We've taken a lot of action as you can imagine in the last 48 hours," he said, adding he would not provide "operation details".

Khan, 28, was shot dead while wearing a fake explosives vest by police on London Bridge after a stabbing spree that also injured three people launched at a nearby prisoner rehabilitation event he was attending. Members of the public were hailed as heroes for preventing even greater loss of life by tackling him - one armed with a five-foot (1.5-metre) narwhal tusk and another with a fire extinguisher.

The incident comes two years after Islamist extremists in a van ploughed into pedestrians on the bridge before attacking people at random with knives, killing eight people and wounding 48.