Prime Minister Theresa May said Britain must be tougher in stamping out Islamist extremism after attackers killed at least seven people by ramming a van into pedestrians on London Bridge and stabbing revelers in nearby bars, Reuters reported.
After the third militant attack in Britain in less than three months, May said Thursday's national election would go ahead. But she proposed regulating cyberspace and said Britain had been far too tolerant of extremism.
"It is time to say enough is enough," the Conservative leader said outside her Downing Street office, where British flags flew at half-staff.
"We cannot and must not pretend that things can continue as they are," May said, adding that Britain was under attack from a new breed of crude copycat militants.
Islamic State, which is losing territory in Syria and Iraq to an offensive backed by a U.S.-led coalition, said its militants were responsible for the attack, the group's media agency Amaq said in a statement monitored in Cairo.
One French national and one Canadian were among those killed. At least 48 people were injured in the attack. Australia said one of its citizens was among the injured.
Police shot dead the three male assailants in the Borough Market area near London Bridge within eight minutes of receiving the first emergency call shortly after 10 p.m. (2100 GMT).
Mark Rowley, head of counter-terrorism police, said eight officers had fired about 50 bullets to stop the attackers, who appeared to be suicide bombers because they were wearing what turned out to be fake suicide vests.
"The situation these officers were confronted with was critical: a matter of life and death," Rowley said. "I am humbled by the bravery of an officer who will rush towards a potential suicide bomber thinking only of protecting others."
A member of the public received non-critical gunshot wounds. Police did not release the names of the attackers.
London police arrested 12 people in the Barking district of east London in connection with the attack and raids were continuing there, the force said. A Reuters photographer saw another raid take place in nearby East Ham.
Less than two weeks ago, a suicide bomber killed 22 children and adults at a concert by U.S. singer Ariana Grande in Manchester in northern England. In March, in a attack similar to Saturday's, five people died after a man drove into pedestrians on Westminster Bridge in central London and stabbed a policeman.
May said the series of attacks were not connected in terms of planning and execution, but were inspired by what she called a "single, evil ideology of Islamist extremism" that represented a perversion of Islam and of the truth.
She said this ideology had to be confronted both abroad and at home, adding that the internet and big internet companies provided the space for such extremism to breed.
Facebook said it wanted to make its social media platform a "hostile environment" for terrorists. Twitter also said it was working to tackle the spread of militant propaganda.
After the Manchester attack, Britain raised its threat level to "critical" - meaning an attack is expected imminently - but downgraded it back to "severe", which means an attack is highly likely, on May 27.