Police were tonight seeking two men wearing crash helmets who fled on foot after a man was shot dead in a jail car park.
Emergency services were called to Wandsworth Prison, south west London, at about 2.30pm after reports that a man had been shot, a Metropolitan Police spokesman said.
The victim, thought to have been a visitor in his 20s who was on his way into the jail, was pronounced dead at the scene on Heathfield Road, the spokesman said.
"Two men wearing crash helmets were seen fleeing from the scene towards Alma Terrace," he added.
Officers from Operation Trident, which investigates gun crime in the black communities, are investigating.
Reports said that at least four shots were fired.
One eyewitness said two men wearing motorcycle helmets with dark visors covering their faces were acting suspiciously nearby before the shooting.
The woman, who asked not to be named, said the two men appeared to be waiting in the street outside the prison entrance.
She said: "I saw two men in the street. They made me nervous. They were wearing motorcycle helmets with the fronts pulled down over their faces.
"They were apart from each other in the street right outside the entrance. One was sitting on a wall.
"They were both wearing white shirts and dark jackets. I walked into a shop. I thought they were cash van security guards.
"That is when I heard several bangs. I thought they were gunshots. There were at least four. One of the staff members said there had been a shooting.
"I could hear sirens. By the time I left the shop there were police and ambulances everywhere."
Police taped off a large part of the leafy, suburban road surrounding the car park.
Residents of the well-heeled London neighbourhood spoke of their shock at the violence.
Scott Russell, 23, said: "From what I have heard it seems to be gangland-related.
"I'm quite shocked because you are not really aware something like this could actually happen outside the prison."
Mr Russell, a university worker who lives close to the car park, went on: "In a way, you think, because you are near the prison, crime would be away from the prison itself.
"This is pretty much a family, suburban area. You would probably think it would be happening in a council estate, but then again crime happens anywhere."
Sheryl Jn-Louis, who had been visiting someone, said: "One of the prison officers told me someone had been shot and wouldn't survive.
"He had been shot in the forehead.
"I have been waiting for an hour because I couldn't get out of the prison."
Janice Dyson, 50, from Ladbroke Grove, was at the prison waiting for a visitor's appointment when she heard shots being fired.
"It was about 2.30 in the afternoon and we had been called up to go and book in our visits," she said.
"I had just got up the steps and I heard four loud bangs. It was four short, very quick shots. A lady, who was waiting for her partner to be released, said there had been a shooting.
"The officer looked out of the window and said that everybody should go in and he shut the door (to the prison).
"People said he (the gunman) was on a motorbike and must have just driven up. I didn't see the person on the motorbike but apparently he was wearing a suit - a smart suit - and a red helmet.
"People who witnessed it were quite hysterical, people screaming, kids crying. I was just trying to calm people down.
"I have heard that the victim was possibly visiting someone in there. I imagine the gunman would be somebody who knew he was going to be there."
She added she believed "it wasn't just a chance".
Police asked anyone with information to call Crimestoppers anonymously on 0800 555111.
The spokesman said officers believed they knew the man's identity, but had yet to inform next of kin.
Wandsworth is one of the largest prisons in England and Wales. Built in 1851, it can hold up to 1,644 category B prisoners.