Emergency services have been called to the Palace of Westminster, where a man has scaled one of the buildings carrying a Palestinian flag.
Video shared on social media shows a barefoot man stood on a ledge several metres up Elizabeth Tower, which houses Big Ben.
The Met Police said officers were called to the tower at 07:24 GMT on Saturday.
“Officers are at the scene working to bring the incident to a safe conclusion,” the force said.
Bridge Street, which is at the north end of Westminster Bridge, is closed to allow the emergency services to deal with the incident, police confirmed.
A spokesman for London Fire Brigade (LFB) said they were responding to the incident alongside the police.
Crews from Lambeth, Chelsea, Soho and Islington fire stations have been deployed, LFB said.
