David Amess, who has been MP for Southend West since 1997 in Essex, was attacked on Friday at Belfairs Methodist Church in Leigh-on-Sea.
An eyewitness said that a man walked into his constituency surgery and stabbed him multiple times.
“We were called to an address in Eastwood Road North shortly after 12.05 pm today. We attended and found a man injured. He was treated by emergency services but, sadly, died at the scene,” Essex Police said in a statement.
“A 25-year-old man was quickly arrested after officers arrived at the scene on suspicion of murder and a knife was recovered. He is currently in custody,” the statement said.