Ten more people have been arrested at a pro-Palestine march in central London, bringing the total to 11.
Officers have made 10 further arrests during an incident at Hyde Park Corner.
Earlier, the Met Police said a man who was seen in the crowd with an antisemitic placard was arrested for inciting racial hatred.
When officers went in to arrest him they were assaulted, resulting in six arrests for assaulting an emergency worker, the force said.
Two other people were arrested for refusing to remove face coverings when required to do so by officers.
A pro-Palestinian car convoy was also stopped in Neasden, northwest London.
Police said in previous years, convoys have driven through areas with significant Jewish communities causing fear for residents, so specialist traffic officers and a police helicopter is closely monitoring the scene.
Police estimated that 30,000 people gathered for the demonstration in central London.
The first arrest was made on suspicion of support for a proscribed organisation in relation to a placard ahead of the start of the march at 1.30pm, police said.
Police say an order is in place until 8pm in much of central London which requires anyone wearing a face covering for the purpose of concealing their identity to remove it when asked to do so.
There is also a dispersal order in place until 11pm, meaning anyone refusing a direction to leave the area can be arrested.
Around 1,500 police officers from forces across the UK are on the streets of London, with the protesters going along Park Lane, Knightsbridge and Kensington Road, then ending at the junction with Kensington Court, where speeches will be made.
According to the Campaign Against Antisemitism, organisers advertised a start time of 12.30pm, but Met Police said starting at 1.30pm would allow time for a synagogue event to finish.
Speakers will address crowds near the Israeli embassy and they must stop by 5pm, while protesters must leave by 6pm, police said.
The Met said "there will be some who ask why" a decision to allow the protest so close to the embassy was allowed but that it is a "common misconception" that forces can allow or refuse permission for a protest to take place.
There must be "a real risk of serious disorder" sufficient for police to request that the Home Secretary ban the protest, which the Met said was not expected on Saturday.
A number of further restrictions are being implemented by police during the protest.
Protesters will be kept more than 100m away from the embassy grounds, behind barriers controlled by officers, and face arrest if they do not do so.
Precautions have also been taken to ensure the presence of protesters does not unnecessarily disrupt other sensitive premises, including synagogues, either near the start or along the route.
Police have also said no gazebos or other stalls can be erected in a specified area at Marble Arch and that anyone participating in the march must not deviate from the route.
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Ahead of the protest, Commander Kyle Gordon, who is lead the policing operation, said: "We are there to ensure protests take place lawfully, minimising disruption to the life of the wider public and in a way that gives due consideration to the cumulative impact on London's communities and those who feel most vulnerable in the current climate.
"The protests we have seen since October have thankfully been largely peaceful and we must take this into account in our policing approach. I hope the same will be true this Saturday.
"Unfortunately, despite this, we have regularly seen officers having to deal with offences related to placards and other hate speech.
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"We do not underestimate the fear this causes, nor the impact of such criminal and unacceptable behaviour on wider community relations," Commander Gordon added.
"I would appeal to all those attending the protest on Saturday to act within the law and to consider the impact of their actions on the safety and security of others. We will not hesitate to take action against those who fail to do so."