LONDON — A news agency affiliated with Islamic State claimed Sunday that the militant group carried out the latest deadly attack in the heart of London, according to the SITE monitoring group, as British Prime Minister Theresa May called for tougher measures to combat Islamist extremism.

Scotland Yard, meanwhile, reported 12 arrests in connection with the vehicle-and-knife rampage in which seven people were killed and dozens of others were injured.

Forensics technicians scoured for clues and police patrolled the scene of Saturday night’s assault, which began with a van ramming pedestrians on London Bridge and continued with a stabbing rampage by three attackers in the crowded riverside nightlife district of Borough Market.

Even in a country still recovering from a major assault less than two weeks earlier, in the northern city of Manchester, determined resilience seemed to be the order of the day Sunday. London Mayor Sadiq Khan said on Twitter that “we will never be cowed by terrorism.”

A benefit concert Sunday evening in Manchester, a tribute to and benefit for those victims, drew about 50,000 people who appeared to thrill to an opening exhortation: to not be afraid.

As the assault unfolded in London on Saturday night, some customers in pubs and restaurants scattered in panic as the assault unfolded – or fought back by hurling bottles and even chairs at the knife-wielding attackers, witnesses said. Police, whose response has been honed by two previous large-scale attacks over the last three months, shot the three attackers dead within eight minutes of the first distress calls.

Armed police patrol St. Thomas Street in London on Sunday near the scene of the attack on London Bridge and at Borough Market. Expressions of sympathy and solidarity continued to pour in from across the world Sunday. Associated Press/Dominic Lipinski, PA

Police acknowledged Sunday that a bystander had been wounded in the gunfire directed at the attackers. Assistant Police Commissioner Mark Rowley said the civilian’s injuries were not life-threatening.

With the investigation still in its early stages, May – who is facing a general election this week – unleashed tough rhetoric against what she called “a single evil ideology of Islamist extremism” without linking a particular group or network to the latest strike.

“Enough is enough,” she said as expressions of sympathy and solidarity continued to pour in from across the world. At the Vatican, Pope Francis offered prayers for the victims at his weekly Sunday blessing.

In a televised statement, the prime minister said the attack marked a “new trend in the threat we face” – assaults in which both methods and ideology echoed those of previous strikes.

“While the recent attacks are not connected by common networks, they are connected in one important sense. They are bound together by the single, evil ideology of Islamist extremism that preaches hatred, sows division, and promotes sectarianism,” May said.

“It is an ideology that claims our Western values of freedom, democracy and human rights are incompatible with the religion of Islam. It is an ideology that is a perversion of Islam and a perversion of the truth. Defeating this ideology is one of the great challenges of our time.”

Shortly after May spoke, London’s Metropolitan Police Service said officers from its counterterrorism command arrested a dozen people in the east London suburb of Barking in connection with the attack. Police said searches of “a number of addresses” in Barking were continuing.

Emergency services reported that 48 people had been taken to half-a-dozen hospitals across the city, including 21 with critical injuries. The injured included an off-duty police officer and an on-duty member of the transport police, and a number of foreign citizens from countries that included Germany, France, Spain, New Zealand and Australia. Canada said one of its nationals was among the dead.

As in past attacks in other cities, including the truck rampage last summer in the French Riviera city of Nice, establishments and ordinary people opened their doors to those left stranded by disrupted transport in an area popular with visitors and locals alike.

Borough Market, with its alleyways beneath and near London Bridge, is filled by day with specialty food stalls, and by night – particularly on a spring evening like Saturday – with patrons who flock to bars, clubs and restaurants.

There have been more than half-a-dozen terrorist incidents in Britain since 2005.

Less than two weeks earlier, a suicide bombing killed 22 people just after a concert by pop singer Ariana Grande in the northern city of Manchester.

May, in her televised address, said that in addition to the attacks carried out in Manchester and on and near London’s Westminster Bridge on March 22, five attacks had been disrupted in recent months. The Westminster attack, carried out by a British convert to Islam, also made use of a car to ram people on the bridge, and a knife for a subsequent stabbing rampage on the grounds of Parliament.

With Britain’s general election only four days away, the major parties called off campaign events Sunday as a sign of respect. The far-right U.K. Independence Party, known for its anti-immigrant, anti-Muslim stance, said it would continue to campaign to send a message to terrorists, it said.

British Home Secretary Amber Rudd said the country’s terrorist threat level would remain at “severe,” because no perpetrators were believed to be at large. The threat level had been raised to “critical” in the days after the Manchester attack.