LONDON, Oct 16 (Reuters) - The Metropolitan Police has said that the murder of British lawmaker David Amess in Essex on Friday has been declared as a terrorist incident, with the investigation being led by its Terrorism Command.

Amess, 69, from Prime Minister Boris Johnson's Conservative Party, was knifed repeatedly in the attack at about midday in the Belfairs Methodist Church in Leigh-on-Sea, east of London.

The Metropolitan Police, whose counter-terrorism unit are leading the investigation of the incident, said in a statement early on Saturday that they declared the fatal stabbing as a terrorist incident.

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 counter-terrorism 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 in vain 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".

"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.

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.

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