LONDON — After the briefest of pauses in electioneering, Prime Minister Theresa May of Britain found her record on security and terrorism under scrutiny on Monday, in the aftermath of a deadly attack in London over the weekend — the third serious terrorist episode in the country in three months.
Before she replaced David Cameron as prime minister last year, Mrs. May was responsible for security during her six years as home secretary. And opposition politicians are highlighting reductions in the number of police officers, including those who are armed, during her tenure.
“The commissioner of the Metropolitan Police has said that the Met is well resourced, and they are; and that they have very powerful counterterrorism capabilities, and they do,” Mrs. May said at a news conference, where she also came to the defense of Sadiq Khan, the mayor of London, who was mocked by President Trump over his response to the attack. “We have protected counterterrorism policing budgets. We have also provided funding for an increase in the number of armed police officers.”
The focus on terrorism would normally be expected to help the prospects of Mrs. May’s Conservative Party in the general election that will take place on Thursday, but her years overseeing counterterrorism policy represent a political problem, one that has been highlighted even by her former political allies.
“I am so sick of Theresa May blaming others for terror when the system she presided over has obviously failed so lamentably,” Steve Hilton, once a close adviser to Mr. Cameron, wrote on Twitter. Mrs. May, he added in a separate tweet, “should be resigning, not seeking re-election.”
Asked by a reporter on Monday if he would support calls for Mrs. May’s resignation over the falling number of police officers, Jeremy Corbyn, leader of the opposition Labour Party, replied, “Indeed, I would.”
The main political parties suspended campaigning on Sunday as a sign of respect for the seven people killed and the scores wounded when three men drove a van into pedestrians on London Bridge on Saturday night and then stabbed people in Borough Market nearby.
As campaigning resumed around the country on Monday, the police continued to investigate associates of the assailants, whose identities are known to the police but have not been made public because the inquiry is continuing.
Twelve people were arrested in Barking, in East London, on Sunday, although one, a 55-year-old man, was released without charges. Early Monday, the police raided two other homes in East London, one in Newham and the other in Barking.
The victims of the attack are believed to have come from several countries, but only one has been identified publicly: The premier of British Columbia in Canada, Christy Clark, confirmed the death of Christine Archibald.
Four police officers were among the wounded. Although there has been widespread praise for the professionalism and courage of the armed officers who shot and killed the assailants within eight minutes of being called Saturday night, the country’s broader antiterrorism strategy was questioned.
Opposition politicians focused their fire on Mrs. May, who gave a short speech outside her office on Downing Street on Sunday saying that “enough is enough,” promising to shake up antiterrorism and deradicalization policies, and calling for new efforts to curb the dissemination of extremist materials on the internet.
Some of Mrs. May’s political opponents regarded her comments as political and, as a result, in breach of the agreement to suspend campaigning.
Late Sunday, Mr. Corbyn criticized the decrease in the number of police officers since 2010. “You cannot protect the public on the cheap,” he said. “The police and security services must get the resources they need, not 20,000 police cuts.”
The total number of police officers in England and Wales fell more than 19,500 from September 2010 to September 2016, according to the Home Office. Authorized firearms officers — or armed police officers — declined to 5,639 in March 2016 from 6,976 in March 2010.
The government says, however, that the number of armed officers is to increase by more than 1,000 in the next two years, that additional specialist teams are being set up outside London, and that there will be 41 additional armed response vehicles.
Mr. Corbyn also accused the government of failing to publish a report, undertaken in early 2016, on foreign financing of extremist groups, for fear of upsetting foreign governments, although he is also vulnerable on security issues. He has demonstrated past support for Irish republicans and expressed doubts two years ago about a so-called shoot-to-kill policy for police officers during serious terrorist attacks.
Yvette Cooper, a Labour lawmaker and former chairwoman of the Home Affairs select committee, told the BBC that it was “inappropriate and wrong” to draw “precise links” between police numbers and individual attacks. But she said that fewer officers made it more difficult to gather information and to counter threats.
In a series of interviews with the news media, Karen Bradley, the culture secretary, declined to answer when challenged about the decline in the number of armed police officers in recent years. She focused instead on the need to increase cooperation with internet service providers to deprive extremists of a safe space online.
Despite the sniping between Britain’s two major parties, Mrs. May had praise for one prominent Labour figure — Mr. Khan. Asked at the news conference about a tweet from Mr. Trump, Mrs. May said she was working closely with Mr. Khan and that he was doing “a good job,” adding that it was “wrong to say anything else.”
The dispute between the American president and the London mayor began with a message from Mr. Khan after the attacks. “Londoners will see an increased police presence today and over the course of the next few days,” he said. “No reason to be alarmed.”
Mr. Trump commented on Twitter with a post suggesting that Mr. Khan was minimizing the threat, but he appeared to misrepresent what the mayor had written. “At least 7 dead and 48 wounded in terror attack and Mayor of London says there is ‘no reason to be alarmed!’ ” he wrote.
A spokesman for Mr. Khan, in response, said on Sunday that the mayor had “more important things to do than respond to Donald Trump’s ill-informed tweet that deliberately takes out of context his remarks, urging Londoners not to be alarmed when they saw more police — including armed officers — on the streets.”
On Monday, Mr. Trump fired back on Twitter, aiming at both the mayor and the mainstream media. “Pathetic excuse by London Mayor Sadiq Khan who had to think fast on his ‘no reason to be alarmed’ statement,” he wrote. “MSM is working hard to sell it!”
Tim Farron, leader of the Liberal Democrats, described Mr. Trump as “an embarrassment to America,” and he called on Mrs. May to cancel his planned state visit to Britain.