Conservative MP Sir David Amess has been stabbed as he met constituents at a regular surgery.
Essex Police said they were called to reports of a stabbing in Leigh-on-Sea at 12:05 BST and arrested a man.
Police recovered a knife and said they were not looking for anyone else in connection to the incident.
Former party leader Sir Iain Duncan Smith said he was "praying for a full recovery".
He said on Twitter: "My thoughts are with David Amess MP and his family at this awful time.
"Praying for a full recovery following this appalling, shocking news. This angry, violent behaviour cannot be tolerated in politics or any other walk of life."
The 69-year-old, who is MP for Southend West, was stabbed as he met constituents at Belfairs Methodist Church.
An air ambulance was sent to the scene.
Southend councillor John Lamb, who was at the scene after the stabbing, said Sir David was a family man, with four daughters and a son.
"He's always trying to help people, and especially refugees he's tried to help. He's a very amicable person and he does stick by his guns, he says what he believes and he sticks by it," Mr Lamb said.
He told the BBC the MP had not been taken to hospital but was operated on by medics at the scene.
Mr Lamb said he was still waiting to hear the extent of Sir David's injuries, but understood the MP was in a "very serious" condition.
The Jo Cox Foundation, the charity set up in memory of the MP who was murdered in 2016, said it was "horrified" by the stabbing.
"We are thinking of him, his family and loved ones at this distressing time," the foundation said.
Labour Party leader Sir Keir Starmer said he was thinking of Sir David, his family and his staff after the "horrific and deeply shocking news".
Essex Police said they were called to reports of a stabbing in Leigh-on-Sea at 12:05 BST and arrested a man.
Police recovered a knife and said they were not looking for anyone else in connection to the incident.
Former party leader Sir Iain Duncan Smith said he was "praying for a full recovery".
He said on Twitter: "My thoughts are with David Amess MP and his family at this awful time.
"Praying for a full recovery following this appalling, shocking news. This angry, violent behaviour cannot be tolerated in politics or any other walk of life."
The 69-year-old, who is MP for Southend West, was stabbed as he met constituents at Belfairs Methodist Church.
An air ambulance was sent to the scene.
Southend councillor John Lamb, who was at the scene after the stabbing, said Sir David was a family man, with four daughters and a son.
"He's always trying to help people, and especially refugees he's tried to help. He's a very amicable person and he does stick by his guns, he says what he believes and he sticks by it," Mr Lamb said.
He told the BBC the MP had not been taken to hospital but was operated on by medics at the scene.
Mr Lamb said he was still waiting to hear the extent of Sir David's injuries, but understood the MP was in a "very serious" condition.
The Jo Cox Foundation, the charity set up in memory of the MP who was murdered in 2016, said it was "horrified" by the stabbing.
"We are thinking of him, his family and loved ones at this distressing time," the foundation said.
Labour Party leader Sir Keir Starmer said he was thinking of Sir David, his family and his staff after the "horrific and deeply shocking news".
