Britain's Prime Minister Theresa May speaks outside 10 Downing Street after an attack on London Bridge and Borough Market left 7 people dead and dozens injured in London, Britain, June 4, 2017. [Photo/Agencies]

LONDON - After a militant attack on a nightlife district of London this weekend, British Prime Minister Theresa May will resume campaigning on Monday just three days before a national election which polls show is much tighter than previously predicted.

May said Britain must be tougher in stamping out Islamist extremism after three knife-wielding assailants rammed a hired van into pedestrians on London Bridge and stabbed others nearby, killing seven people and injuring 48.

After the third militant attack in Britain in less than three months, May said Thursday's election would go ahead. But she said Britain had been far too tolerant of extremism.

"Violence can never be allowed to disrupt the democratic process," May said outside her Downing Street office, where British flags flew at half-staff.

Islamic State on Sunday night claimed responsibility for the attack via the militant group's agency Amaq.

"A detachment of Islamic State fighters executed yesterday's London attack," a statement posted on Amaq's media page, monitored in Cairo, said.

A policewomen takes away a suspect in connection with the attack during a police raid of the Barking district of east London, June 4, 2017. [Photo/Xinhua]

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. Police have not released the names of the attackers.

It was not immediately clear how the attack would impact the election. The campaign was suspended for several days last month when a suicide bomber killed 22 people at a concert by Ariana Grande in Manchester.

Grande gave an emotional performance on Sunday at a benefit gig in the city for the victims of the attack, singing with a choir of local schoolchildren, including some who had been at her show.

Before the London Bridge attack, May's gamble on a June 8 snap election had been thrust into doubt after polls showed her Conservative Party's lead had collapsed in recent weeks.