Police carried out fresh raids and arrested "a number of people" on Monday after the Islamic State group claimed an attack by three men who mowed down and stabbed revelers in London, killing seven people before being shot dead by officers.

Saturday night's rampage at a popular nightlife hub around London Bridge by men wearing fake suicide vests was the third deadly terror attack in Britain in less than three months and came only days before a general election.

"A number of people have been detained," police said in a statement after two early morning raids in east London, as commuters returned to the scene of the attacks after some security cordons were removed.

"A very high priority for us is to try to understand whether they were working with anybody else," London police chief Cressida Dick told BBC television. Dick said police had seized "a huge amount of forensic material" after going through the van used in the attack "very very carefully". "We will change and adapt to what appears to be a new reality for us," she said.

British Prime Minister Theresa May on Sunday blamed "evil" Islamist ideology and vowed to crackdown on extremist content online worldwide, warning that attackers were "copying one another".

National campaigning for Thursday's general election resumed on Monday after a one-day suspension out of respect for the victims, who included 48 people treated in hospital for injuries. Of those, 21 are still in a critical condition.

Police said on Sunday they were holding 11 people, all arrested in raids on two addresses in Barking in suburban east London. The Islamic State group claimed responsibility.

No details have been released about the perpetrators, who were shot dead within eight minutes of the first call to the police.