LONDON — The authorities in Britain said on Monday that they were treating an early morning attack near a mosque in London as a possible act of terrorism, amid fears of retaliation for several recent assaults in the country attributed to Islamist extremists.
Shortly after midnight, a van rammed into a group of pedestrians near the Finsbury Park Mosque, in North London. One person died at the scene and at least 10 were wounded, but the authorities said it was not immediately clear if the attack was responsible for the man’s death.
The assault, which the police said was carried out by a 48-year-old white man who was believed to be acting alone, drew condemnation from a broad array of political and religious figures and came at what Prime Minister Theresa May acknowledged was “a difficult time in the life of the city.”
Some Muslims and others were asking whether the authorities and the news media were quick enough to describe the assault on Monday as an act of terrorism — although Mrs. May said it had been declared terrorism within eight minutes.
Speaking in front of her offices at 10 Downing Street, Mrs. May denounced the assault as an act of “hatred” and “evil” against innocent civilians during the holy month of Ramadan, and said security at mosques would be bolstered.
Seeking to capture the mood of a wounded but unbowed capital, Mrs. May said that it was an “extraordinary city” of “extraordinary people,” and that British values of freedom of speech and freedom of religion would prevail.
Calling the attack a “sickening attempt” to destroy those freedoms, she noted that extremism and hatred could take many forms. But she said the country would never give into hate.
Harun Khan, secretary general of the Muslim Council of Britain, said he recognized that many Muslims would be angry about the attack and appealed for calm, but he also warned of a rising tide of anti-Islamic sentiment.
“During the night, ordinary British citizens were set upon while they were going about their lives, completing their night worship,” Mr. Khan said. “Over the past weeks and months, Muslims have endured many incidents of Islamophobia, and this is the most violent manifestation to date.”
Deputy Assistant Commissioner Neil Basu, the senior national coordinator for counterterrorism at the Metropolitan Police, praised bystanders who had intervened to detain the suspect, and he urged residents to remain calm and vigilant.
“No matter what the motivation proves to be, and we are keeping an open mind, this is being treated as a terrorist attack and the Counter Terrorism Command is investigating,” he said, adding that additional officers had been deployed across London.
Britain has been shaken by a series of attacks recently. In March, an assailant mowed down pedestrians on Westminster Bridge and stabbed a police officer outside Parliament, killing five people before the police shot and killed him.
In May, at least 22 people were killed at an Ariana Grande concert in Manchester, England, including many teenagers and an 8-year-old girl.
In early June, at least eight people were killed and dozens more wounded after two men sped across London Bridge in a white van, ramming numerous pedestrians before emerging with large hunting knives for a rampage in Borough Market, a crowded nightspot.
In addition, there has been widespread anger after a fire tore through a 24-story public housing high-rise, killing at least 79 people, and provoking widespread accusations that the fire could have been prevented.
Residents of London, a multicultural city with a large Muslim population, have predominantly responded with equanimity, solidarity and tolerance toward Muslims following recent attacks perpetrated by extremists. On Monday, a similar sense of unity prevailed.
“We want to recognize this as an incident the same as other incidents,” said Deb Hermer, a 20-year resident of Finsbury Park who left a bouquet of flowers at the gate of the mosque. “This is no less important than other incidents.”
But Muslim leaders and human rights advocates have warned that some could try to use recent terrorist attacks to try to stir hostility against Muslims, and to foster the notion of a culture war between Islam and the West.
According to the office of the mayor of London, in the six days after the terrorist attack at London Bridge and Borough Market on June 3, the Metropolitan Police reported 120 Islamophobic events, compared with 36 the previous week. It added that hate crimes in general had been growing.
“We call on politicians to treat this major incident no less than a terrorist attack,” the association said in a statement. “We call on the government to do more to tackle this hateful evil ideology, which has spread over these past years and resulted in an increase of Islamophobic attacks and division of our society, as well as spreading of hate.”
Brendan Cox, whose wife, Jo Cox, a member of Parliament, was shot and killed last year in northern England by a right-wing extremist, said it was imperative to battle hateful ideology against Muslims, just as it was necessary to fight Islamist hate preachers.
“When islamist terrorists attack we rightly seek out hate preachers who spur them on,” Mr. Cox wrote on Twitter. “We must do the same to those who peddle Islamophobia.”
J. K. Rowling, author of the “Harry Potter” books, criticized The Daily Mail for the way it referred to the assailant. “The Mail has misspelled ‘terrorist’ as ‘white van driver,’ ” she wrote on Twitter. “Now let’s discuss how he was radicalized.”
The driver of the van on Monday was arrested after bystanders prevented him from fleeing, the police said in a statement. Commissioner Basu of the Metropolitan Police praised the witnesses who had intervened, saying they had responded quickly and calmly to the police even while they were shaken, scared or angry.
He said the police had received a number of calls reporting that a van had rammed into pedestrians, and that officers in the area had responded instantly.
Aweys Sheikh, 45, said that he was walking home from the Muslim Welfare House on Seven Sisters Road shortly after midnight when he heard a noise and people screaming. “Then I saw the back of the van,” he said, and he ran to help.
“I saw the man who came out of the van, he tried to escape so I held him down with two more boys,” he said.
Mahroof Mohammed, another witness, said he was having his evening tea at a Somali restaurant on Seven Sisters Road when he heard people running.
He went outside and saw several injured people. “There were seven or eight. Three of them were bleeding badly,” he said. “They were all leaving the mosque when they got hit.”
Mr. Mohammed said that most of the victims he had seen were men, but that one was an older woman, and he described seeing “three local men that were holding the man from the van until police came.”
“I saw the attacker attempting to run away, but people from the mosque held him back,” said another witness, Boubou Sougou, 23. “Some of them wanted to beat him up, but were stopped by the ones that were holding him until the police came.”
There were reports that other people had fled from the van after the attack, but Commissioner Basu seemed to discount that theory, saying there were no other suspects.
By midmorning, police officers had cordoned off a corner of Seven Sisters Road, and news cameras swarmed nearby. Police vans that had been on site began to leave, though traffic on a nearby major thoroughfare, Blackstock Road, was restricted.
The Finsbury Park Mosque opened in 1994 and became a hotbed of Islamist militants, including Zacarias Moussaoui, a Frenchman convicted of conspiring to kill Americans as part of the Sept. 11, 2001, attacks, and Richard C. Reid, who tried to down an American jetliner in late 2001 with explosives packed in his shoes.
In 2015, the mosque’s former imam, Mostafa Kamel Mostafa, was sentenced to life in prison in Federal District Court in Manhattan on 11 terrorism-related charges.
The mosque was raided by the authorities in January 2003, and in February 2005 it was reconstituted — “run by a new board of trustees with a new management team, new imams, a new name and new ethos,” according to its website. Five stories tall with space for 1,800 worshipers, it is a major house of worship for North London, an area known for a large immigrant population.
Since then, the mosque has worked to shed itself of its radical links, and on Monday, Christian ministers gathered there to show solidarity.
“There are very good friendships between the Christian and Muslim communities here,” said the Rev. Daniel Sandham of St. John the Evangelist, a nearby church. “It has sought to be a beacon for the community.”