Armed police were tonight in a stand-off with a gunman after shots were fired on a busy street.
The man was in a house in Markham Square, Chelsea, west London, and police negotiators had been sent to the scene, a Met spokesman said.
Television pictures showed officers at the door of a home in the square.
Officers were called at 4.50pm today after reports of gunshots. They were shot at and returned fire.
So far there are no reported injuries and no one has been arrested.
One eyewitness said there was an argument between a man and a woman before the shooting started.
At 9.32pm loud bangs were heard coming from the area were the siege is taking place.
One onlooker said: "There were a series of very loud bangs. It sounded like gunfire. Maybe four or five shots. It was loud enough to hear over the helicopter flying above us."
Commander Ali Dizaei, who is in overall command of the operation, confirmed that further gunfire was exchanged just after 9pm, and that police had returned fire. However, there were no injuries reported, and Commander Dizaei said negotiations were in progress to bring the matter to a "safe conclusion".
A police spokeswoman said there were not thought to be any hostages involved.
PA