(The Washington Post)

Less than 24 hours after terrorists killed seven people and injured dozens more in the heart of London, the latest attack to hit Britain this spring became a campaign issue Sunday, with just four days before an unpredictable national election. 

Rival party leaders lashed out at one another as police raided homes and carried out a dozen arrests, and as the nation mourned. Tens of thousands attended an Ariana Grande benefit concert that was originally intended to honor the dead from last month’s suicide bombing in Manchester but was expanded to recognize the newest victims in London.

Following the May 22 attack in Manchester, Saturday night’s van-and-knife rampage was the second mass-casualty attack to intrude on the homestretch of a parliamentary campaign that was once thought certain to end in a landslide for Prime Minister Theresa May and the Conservatives. The race has tightened in recent weeks, and terrorism has introduced an unexpected variable.

With her premiership on the line, May took an aggressive and combative tone Sunday, telling the nation that “enough is enough” and insisting there is “far too much tolerance for extremism in our country.”

“Things need to change,” May said in a speech outside the prime minister’s residence at 10 Downing Street. 

She blamed the attack on the “evil ideology of Islamist extremism,” called for a thorough review of the nation’s counterterrorism policies and suggested she will take a much tougher line if she wins Thursday’s vote. 

The speech was criticized by the opposition Labour Party as a thinly veiled jab at their far-left leader, Jeremy Corbyn, whom May has often accused of coddling anti-Western militants. May, Corbyn’s backers said, had politicized the attack.

But by evening, Corbyn had hit back with his own political response to the killing, accusing May and her Conservative allies of weakening security services through years of austerity. 

“You cannot protect the public on the cheap,” Corbyn said in a speech in the northern English city of Carlisle that ended a brief pause in formal campaigning. “The police and security services must get the resources they need, not 20,000 police cuts.”

Corbyn also derided President Trump, accusing him of lacking both “grace” and “sense” after the U.S. leader twisted a quote from London Mayor Sadiq Khan in order to launch an attack on the West’s most prominent Muslim politician.

May, who has gone to great lengths to cultivate ties with Trump, had earlier defended Khan while carefully avoiding any criticism of the U.S. president.

The multilayered controversy came as investigators were just beginning to unravel details of the assailants and the plot behind the killings, which jolted the country Saturday night.

At just after 10 p.m. that night, three men plowed a rented ­Renault van into a crowd of pedestrians on London Bridge, then got out and used knives to slash bar and restaurant patrons at the nearby Borough Market. 

The attackers were fatally shot by police within eight minutes of the first emergency call, with eight officers firing a total of 50 rounds at men who had donned camouflage and fake suicide vests to carry out the carnage.

British authorities did not identify the victims. But Canada’s prime minister and France’s foreign minister confirmed that their nationals were among the dead. 

Mark Rowley, assistant commissioner of London’s Metropolitan Police, said in a late afternoon news conference on Sunday that investigators were still trying to confirm the identities of the attackers and that they were “increasingly confident” there were no other perpetrators. He said police had “more to do” to determine whether the assailants had help in planning the attack. 

Rowley praised the performance of officers in responding to the attack — a view that was echoed almost universally Sunday — and described the number of shots fired as “unprecedented” in a country where most officers do not carry a firearm and those who do rarely, if ever, use it.

The fusillade, Rowley said, was necessary “to be completely confident [officers] had neutralized the threat that those men posed.”

At least 48 people were injured in the attack — including one bystander who was shot by an errant police bullet and was expected to recover. Four officers were among the injured. Rowley said Sunday that 21 of those injured are in critical condition.

As doctors and nurses tended to the wounded, police carried out raids in the East London neighborhood of Barking in a signal that authorities are probing at least the possibility that others may have been involved in the planning of the attack. A dozen people were arrested, police said.

In Barking, neighbors said police had taken at least five people away early Sunday from a mixed-income, 10-story building believed to have been home to one of the attackers. Neighbors said that they heard loud bangs during the raid and that one of the men who was ultimately arrested had tried to flee.

Even as the investigation intensified, authorities did not raise the nation’s threat level, as they had after the bombing in Manchester last month. The decision suggested authorities did not believe another attack is imminent, though under the existing “severe” rating, one is considered highly likely.

Investigators were focused on the likelihood that the attack had been inspired, if not directed, by the Islamic State, which claimed responsibility Sunday (although similar claims in the past have been shown to be unreliable). The militant group has called on its followers to carry out attacks in the West, especially during the Muslim holy month of Ramadan.

On Sunday night, the SITE Intelligence Group said in a news release that “the pro-Islamic State (IS) Nashir News Agency called for additional revenge attacks against Western states involved in the coalition.” It added that the group “distributed a poster bearing a message in Arabic, English, and French across its Telegram channels on June 4, stating, ‘Revenge – No compromise . . . in the security of Muslims,’ and showing London Bridge, a lorry, and the silhouette of a fighter bearing a knife.”

Saturday’s killings follow both the Manchester attack and a March attack that was eerily similar in style to the one that unfolded at and around London Bridge. In March, an attacker rammed pedestrians on a different Thames River crossing and stabbed to death a police officer at the gates of Parliament. 

The three recent attacks were not connected, May said. But she described it as “a new trend” in which terrorists are “copying one another and often using the crudest means of attack.”

May did not detail her plans for confronting the threat. But she floated the idea of tougher prison sentences for less serious terrorism-related offenses and called on tech companies to do more to crack down on extremist content online.

Facebook responded with a statement calling for “strong partnerships” between tech firms and policymakers.

May also seemed to acknowledge Sunday that British security services are struggling to keep up as the scale of the threat grows. The services say they have disrupted at least 18 plots in recent years. But they have about 3,000 suspected extremists on watch lists — far too many to actively monitor at all times. 

Previous attacks have been carried out by people who had been flagged to the security services for concern but had been judged to be peripheral to any active plots.

May had returned from the campaign trail to 10 Downing Street late Saturday for emergency meetings with security officials. On Sunday morning, all the major parties, including May’s Conservatives, suspended campaigning.

Amid speculation that the election could be postponed, May quickly announced that it would go ahead as scheduled, a position that was endorsed by her rivals. Corbyn told Sky News that “democracy must prevail. If we allow these attacks to disrupt our democratic process, then we all lose.”

Adding to the growing political debate over the attack were Sunday morning tweets by Trump, who took aim at political correctness, the push in the United States for tougher gun laws and Khan, London’s mayor. Trump chided Khan for attempting to calm the public by assuring that there was “no need to be alarmed.” 

Khan’s comments were in reference to an escalated police presence on London streets. But Trump incorrectly implied they were a comment on the attack itself. 

Khan’s office released a statement saying the mayor “has more important things to do than respond to Donald Trump’s ill-informed tweet.”

Trump’s tweets were widely mocked in Britain, where the overwhelming mood was one of unity against terrorism and praise for security services.

The police cordon around London Bridge and Borough Market was gradually reduced Sunday as forensics teams continued to secure evidence and heavily armed officers guarded empty streets. On several streets leading toward Borough Market, tourists and London residents had dropped flowers. Signs reading “ISIS will lose. #Love will win” were attached to street signs.

On Sunday evening, Grande, the pop star who performed in Manchester on the night of the concert attack, sang again in a charity concert to benefit the victims. 

“I love you guys so much,” an emotional Grande told the crowd of 50,000 that had packed into a cricket ground amid extraordinary security. “This night is the kind of thing the world really needs right now.”

William Booth and Rick Noack contributed to this report.