Emergency services have cordoned off the area surrounding Big Ben after a man scaled one of the buildings in the Palace of Westminster.
Video on social media shows a barefoot man stood on a ledge holding a Palestinian flag several metres up Elizabeth Tower, which houses the Westminster clock and its world-famous bell.
At least nine emergency service vehicles lined Bridge Street in central London and crowds looked on from beyond a police cordon.
A Met police spokesperson said: “At 7.24am on Saturday 8 March officers were alerted to a man climbing up the Elizabeth Tower at the Houses of Parliament.
“Officers are at the scene working to bring the incident to a safe conclusion. They are being assisted by the London Fire Brigade and the London Ambulance Service.”
Bridge Street, which is at the north end of Westminster Bridge, is closed to allow the emergency services to deal with the incident, police said.
A spokesman for London Fire Brigade said: “Firefighters are responding alongside the Metropolitan Police Service to reports of a person scaling the Palace of Westminster.”