Prime Minister Theresa May responded on Sunday to an attack in London which killed seven people, declaring "enough is enough” and calling for a review of Britain's counter-terrorism strategy.

May, who faces a general election Thursday, said the recent attacks are not directly connected but that "terrorism breeds terrorism" and attackers copy one another.

She also said five credible plots have recently been disrupted.

"They are bound together by the single evil ideology of Islamist extremism that preaches hatred, sows division, and promotes sectarianism," she said.

"It is an ideology that claims our Western values and freedom, democracy and human rights are incompatible with the religion of Islam," she warned.

"We believe we are experiencing a new trend in the threat we face, as terrorism breeds terrorism, and perpetrators are inspired to attack not only on the basis of carefully-constructed plots after years of planning and training and not even as lone attackers radicalised online but by copying one another and often using the crudest of means of attack."

She called for international agreements to regulate cyberspace to help stop the spread of extremism and said Britain needs to become more robust at identifying and stamping it out.

"That will require some difficult, and often embarrassing, conversations," she said.

"Defeating this ideology is one of the great challenges of our time. But it cannot be defeated through military intervention alone. It will not be defeated through the maintenance of a permanent, defensive counter-terrorism operation, however skillful its leaders and practitioners. It will only be defeated when we turn peoples minds away from this violence."

The violence turned a summery night in an area packed with revellers into a scene of panic and chaos, with officers running through crowded streets screaming for people to flee.

Lifeboats on the River Thames helped evacuate the area, which is popular with tourists.

It remained closed off Sunday and police urged residents and tourists to stay away.

May said the men attacked "innocent and unarmed civilians" in Borough Market with blades and knives.

She said they were wearing what appeared to be explosive vests, but police determined those were only meant to sow panic and fear.

"If we need to increase the length of custodial sentences for terrorism-related offences, even apparently less serious offences, that is what we will do."

May's Conservative Party had been expected to win by a wide margin but recent polls have showed the race tightening.

Major parties temporarily suspended national campaigning Sunday. However, May said, "those campaigns will resume in full tomorrow. And the general election will go ahead as planned on Thursday."

London Mayor Sadiq Khan said the attackers hoped to disrupt democracy and hamper plans for voting, but that would not happen.

He said Londoners should remain vigilant but added: "I'm reassured we are one of the safest global cities in the world."

Assistant Metropolitan Police Commissioner Mark Rowley said that while police believe all the attackers were killed, the investigation continues.

"We believe three people were involved, but we still have got some more inquiries to do to be 100 per cent confident in that," he said.

As dawn broke over the capital, a large area on the south bank of the River Thames remained cordoned off. Police told people to avoid the area, leaving tourists and revellers struggling to get home.

Khan said some of the injured were in critical condition. French and Spanish citizens were among the wounded.

Read the full text of May's statement here.