The Metropolitan Police issued a statement shortly after midnight:
The fatal stabbing in Leigh-on-Sea has tonight been declared as a terrorist incident, with the investigation being led by Counter Terrorism Policing.
The investigation is being led by the Met’s Counter Terrorism Command who are working closely with colleagues from the Eastern Region Specialist Operations Unit (ERSOU) and Essex Police.
Senior National Coordinator for Counter Terrorism Policing, Deputy Assistant Commissioner Dean Haydon formally declared the incident as terrorism. The early investigation has revealed a potential motivation linked to Islamist extremism.
Essex Police responded to an incident at an address in Eastwood Road North, Essex, shortly after 12:05hrs on Friday, 15 October.
At the scene, officers found a man with multiple stab wounds. He was given emergency medical treatment by emergency services, but sadly died at the scene.
The man was identified as Sir David Amess, Member of Parliament for Southend West. Specialist officers are supporting his family.
A 25-year-old British man was arrested at the scene on suspicion of murder. He is currently in custody at a police station in Essex.
As part of the investigation, officers are currently carrying out searches at two addresses in the London area and these are ongoing.
It is believed that he acted alone, and we are not seeking anyone else in connection with the incident at this time. However, enquiries into the circumstances continue.
Detectives would urge any witnesses or anyone with information about this incident to contact police.
If you have any information that could assist the investigation, then please call police in confidence on 0800 789 321. Anyone with moving footage or pictures is asked to submit them via this link