Protesters have been photographed forming a line and holding banners, blocking rush-hour traffic on Haymarket.
The major thoroughfare runs between Piccadilly Circus and Trafalgar Square hosting a number of theatres, hotels and restaurants.
The rally is understood to be moving slowly towards Waterloo.
“Officers are on scene and we’ll aim to get traffic moving again freely as soon as it is safe to do so,” the Met said in a statement.
Buses, lorries and commuters have been forced into a gridlock snaking behind the protesters as they chant, hold placards and walk slowly in a line.
