Police said that the violence occurred after the official demonstration ended, when a group of around 500 out of the 8,000 to 10,000 at the event stayed to protest in violation of police orders. Law enforcement officers began making arrests.
Police said some in the crowd resisted arrest, requiring officers to use force. The protesters then broke away, marching to Bridge Street, outside Westminster tube station, where cordons were placed to detain them.
The 40 people were arrested for offenses including breach of Public Order Act conditions, blocking the highway and assaulting emergency workers.
The demonstration was organized by a coalition of anti-Israel groups, including the Palestine Solidarity Campaign.