Sir. David Amess, MP, was stabbed by a man while he hosted a surgery inside Belfairs Methodist Church.
Essex Police confirmed the reports of a stabbing incident and said officers had arrested one man at the scene.
There are reports that an air ambulance was scrambled to the scene.
“A man’s been arrested following an incident in #LeighonSea,” said Essex Police.
“We were called to reports of a stabbing in Eastwood Rd North shortly after 12.05pm.
“A man was arrested shortly after & we’re not looking for anyone else.
“We’ll bring you more info when we have it.”
Sir. David Amess, who was from Plaistow in east London, was one of the longest currently serving MPs - he was first elected in 1983.