Emergency services have been called to the Palace of Westminster where a man has scaled one of the buildings.
Video on social media shows a barefoot man stood on a ledge holding a Palestine flag several metres up Elizabeth Tower, which houses Big Ben.
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 March 8 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.”
At around 10am on Saturday, three emergency personnel were lifted several metres up on a fire brigade aerial ladder platform with one person using a megaphone to speak to the man on the ledge.
Bridge Street and Westminster Bridge have been closed to allow the emergency services to deal with the incident, police confirmed.
It is understood one exit of Westminster Tube station is closed, but there is no disruption to Tube services and passengers can use other exits.
A spokesman for London Fire Brigade (LFB) said: “Firefighters are responding alongside the Metropolitan Police Service to reports of a person scaling the Palace of Westminster.”
Crews from Lambeth, Chelsea, Soho and Islington fire stations have been deployed, LFB added.
Press Association