LEIGH-ON-SEA, England—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 …