London will not be cowed by militants who have killed 35 people in three separate attacks on the United Kingdom but police will ask for more resources to tackle the threat from marauding assailants, London police chief Cressida Dick told Reuters.

Three Islamist militants rammed a hired van into pedestrians on London Bridge last Saturday before going on the rampage through the bustling Borough Market area where they slit throats and stabbed people.

Three French, two Australians, one Canadian, a Spanish and a Briton were killed in the attack. Armed police shot dead the three attackers just eight minutes after the first phone call from the public.

Metropolitan Police Commissioner Dick said she understood that people were concerned about security after attacks in London and Manchester but that the British capital was a safe city that would not be intimidated.

"London is a safe city and we will do everything we can in our power to keep you safe," she said in an interview at London police headquarters just a few steps away from Westminster Bridge where an attacker killed 5 people on March 22.

"We won't let them win and London will carry on," she said.

After the London Bridge attack, Prime Minister Theresa May said that Britain was under threat from a new breed of crude copycat militants.

"We are highly effective at preventing attacks in this country and we will step up a gear and do our level best to stop any further attacks," Dick said.

"But as you point out, highly volatile people who are intent on doing something absolutely terrible and who are quite happy - maybe even pleased - to kill themselves and to use a low tech methods - these are difficult things to defend against."

When asked if the Metropolitan Police needed more resources she said: "We will want more resources to help us and I am sure the same will be being said in the intelligence agencies."

"But obviously we will be reviewing, for example the number of armed officers we have and how they work, and a whole host of other things that is likely, from my point of view, to take more resources and I will be asking for them."