The murder of Tory MP Sir David Amess is being treated as a terrorist incident.
The Metropolitan Police Counter Terrorism Command is leading the investigation.
Deputy Assistant Commissioner Dean Haydon formally declared the incident as terrorism.
The early investigation has revealed a potential motivation linked to Islamist extremism.
A 25-year-old man was arrested at the scene on suspicion of murder.
He is in police custody in Essex.
Meanwhile, police are carrying out searches at two addresses in London linked to the man.
However, it is believed he was acting alone in the attack in Leigh-on-Sea.
Sir David died after being stabbed “multiple times” at his constituency surgery.
Sir David, 69, was the MP for Southend West in Essex.
The incident happened at the Belfairs Methodist Church.
It is reported Sir David was initially treated at the scene by paramedics but later died from his injuries.