The early investigation has revealed a “potential motivation linked to Islamist extremism,” the police added.

A 25-year-old man was arrested at the scene on suspicion of murder, and detectives said specialist counterterrorism officers were leading the initial investigation.

“As part of the investigation, officers are currently carrying out searches at two addresses in the London area and these are ongoing,” the police said, adding that it is believed that the suspect in custody acted alone.

Politicians described the attack as an assault on democracy.

“David was a man who believed passionately in this country and in its future and we have lost today a fine public servant and a much loved friend and colleague,” said Johnson, who rushed back to London from the west of England after the news broke.

Armed police swooped on the church and paramedics fought to save the lawmaker’s life on the floor of the church – where a sign says “All are welcome here: where old friends meet and strangers feel at home” – but in vain.

“Tragically, he died at the scene,” Essex Police Chief Constable Ben-Julian Harrington told reporters. He said police at the time of the incident did not believe there was any immediate threat to anyone else.

“It will be for investigators to determine whether or not this may have been a terrorist incident. As always they will keep an open mind,” he said.

He gave no other details about the killing, the second fatal attack on a British lawmaker in their constituency in the last five years, which has prompted questions about the safety of politicians.

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Broadcaster Sky News said the arrested man was understood to be a British national of Somali heritage.

Colleagues from across parliament expressed their shock and paid tribute to Amess, one of Britain’s longest-serving lawmakers who held regular meetings with voters on the first and third Friday of the month, saying he was diligent in his duties to his local area.

Flags on all British government buildings will be flown at half-mast in tribute.

Amess, married with five children, was first elected to parliament to represent Basildon in 1983, and then Southend West in 1997. He was knighted by Queen Elizabeth for his public service in 2015.

His website listed his main interests as “animal welfare and pro-life issues”. He was popular with lawmakers and known for his active contributions to debates – often about issues relating to his Essex constituency or animal rights.

In his last contribution to the House of Commons last month, he asked for a debate about animal welfare.

Residents left flowers beside the church with a tribute: “David Amess RIP Such a gentleman xxx.”

The knife attack at a meeting with constituents has echoes of a 2010 incident when Labour lawmaker Stephen Timms survived a stabbing in his constituency office, and the 2016 fatal shooting of Labour’s Jo Cox just days before the Brexit referendum.

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Her husband Brendan called the attack “as cowardly as it gets”, while her sister, Kim Leadbeater, who earlier this year was elected as member of parliament for the same area Cox represented, said it showed the “massive risks” lawmakers had to take.

“That another family is having to go through that again, it’s horrific,” Leadbeater said. “So many MPs today will be scared by this. My partner came home and said: ‘I don’t want you to do this any more, because next time that phone goes it could be a different conversation’.”

The speaker of the House of Commons, Lindsay Hoyle, said the incident would send shock waves across the parliamentary community and the whole country, adding the security of lawmakers would have to discussed.

“Questions are rightly being asked about the safety of our country’s elected representatives and I will provide updates in due course,” Home Secretary Priti Patel said.

US Secretary of State Antony Blinken also conveyed his condolences. “An attack on elected officials is an attack on democracy,” he said.

The Conservative Party suspended all campaigning activities until further notice. Colleagues said Amess was a dedicated parliamentarian and cast him as a true gentleman.

“Heartbreaking to hear of the death of Sir David Amess,” former prime minister Theresa May said. “A decent man and respected parliamentarian, killed in his own community while carrying out his public duties. A tragic day for our democracy.”

Johnson’s wife, Carrie, said she was devastated.

“He was hugely kind and good,” she said on Twitter. “An enormous animal-lover and a true gent. This is so completely unjust.”