An early investigation into the stabbing attack revealed a "potential motivation linked to Islamist extremism."
British lawmaker David Amess was stabbed to death in an Essex church on Friday by an assailant who lunged at him as he met voters, in what police said was a terrorist attack.
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.