2.40pm: Emergency services are called to an incident in Parliament Square. It emerges that an attacker, with two large knives, hit pedestrians with his car on Westminster Bridge, then hit the gates at the front of the Houses of Parliament, stabbing a police officer, who was unarmed, before being shot dead.
MPs are told the chamber would remain in "lockdown" until further notice
Around 3.30pm: Scotland Yard says the attack is being treated as a "terrorist incident until we know otherwise"
4pm: The first death is confirmed when a junior doctor at St Thomas' Hospital says one woman has died and a number of others have "catastrophic" injuries
Before 4.50pm: Commander BJ Harrington of the Metropolitan Police says there are "a number of casualties" in the attack, "including police officers"
Before 5pm: London Ambulance Service say at least 10 patients were treated on Westminster Bridge, with hospitals on alert
Before 5.40pm: Sources say the police officer who was stabbed has died
Around 6pm: Scotland Yard's top anti-terror officer Mark Rowley confirms four people have been killed, including the police officer who was killed, and his attacker
Before 6.45pm: A Number 10 spokesman says the Prime Minister will chair a meeting of the Cobra committee to discuss the response to the incident. She orders flags to be lowered at half-mast over Downing Street as a mark of respect to those who lost their lives.
Around 8.50pm: Theresa May spoke at Downing Street and praised police officers for their "exceptional bravery". She said: "We will all move forward together, never giving in to terror and never allowing the voices of hate and evil to drive us apart."
10.30pm: Police name the officer killed in the attack as Keith Palmer, a 48-year-old husband and father, and say the number of dead has risen to five, including the attacker