Britain's national parliamentary election on Thursday should not be postponed following an attack in London that killed six people late on Saturday, London Mayor Sadiq Khan said.

Khan said Londoners would see an increased police presence in the city after the deadly incident but there was no reason to panic. He urged people to remain calm and vigilant.

Khan also said the official threat level remained at severe, meaning an attack is highly likely.

"One of the things that we can do is show that we aren't going to be cowed, is by voting on Thursday and making sure that we understand the importance of our democracy, our civil liberties and our human rights," Khan said.

"I'm not an advocate of postponing the election. I'm a passionate believer in democracy and making sure that we vote and we recognize actually that one of the things these terrorists hate is voting, they hate democracy," he said.