Three police officers have been injured during a demonstration in Westminster organised by the Palestine Solidarity Campaign and other groups.
One officer suffered a serious facial injury after she was hit with a bottle thrown from within the crowd, the Metropolitan Police said.
The Met said the suspect had not yet been identified but that police were investigating.
40 arrests related to assaults on emergency workers, obstruction of the highway and public order act breaches were made, the force said.
The protest began in Whitehall at 18:00 BST and was due to end two hours later.
Police said the “vast majority” of protesters – between 8,000 and 10,000 people – left without incident at the scheduled time.
A group of about 500 remained and continued to protest.
The Met said some resisted arrest, which required officers “to use force” to remove them from the crowd.
A breakaway march then made its way to Bridge Street, outside Westminster station, where police put cordons in place.
Officers went into the crowd shortly before 22:00 to arrest those suspected of leading the breakaway protest.
The Palestine Solidarity Campaign said it had organised the emergency march to reiterate calls for a ceasefire in Gaza, following an Israeli air strike and resulting fire that killed at least 45 Palestinians on Sunday.
