The Metropolitan Police has said it is "actively looking" for individuals pictured at the pro-Palestinian march in London carrying antisemitic placards.
Hundreds of thousands of people have taken part in a march in the capital calling for a ceasefire in Israel's war against Palestinian militant group Hamas.
While it remained largely peaceful, images are circulating on social media showing some protesters having carried the signs.
Follow latest updates: Tensions remain on London streets after march
The Met Police has responded to a number of the posts saying it was working to identify those involved including individuals who wore what were described as Hamas-style headbands.
"Officers are actively looking for these individuals and will take proactive action when they are identified," the force commented.
The editor of the Jewish Chronicle, Jake Wallis Simons, is among those posting images of the placards, including one he said was pictured in London today showing a snake in the colours of the Israeli flag encircling the world.
He told Sky News that this sign stood out to him, as he claimed it was a "direct link to Nazi Germany".
Mr Simons criticised the police for "failing to uphold the law" during the demonstration, as he said the march featured examples of "old antisemitism repackaged for a modern age".
"(There were) open calls for genocide," he claimed, adding: "I am not asking for the law to be changed, I want the law to be enforced."
As protesters gathered at the start of the route at Hyde Park, chants of "from the river to the sea, Palestine will be free", could also be heard.
The slogan is viewed by many Jews as antisemitic, and is taken to mean a call for the eradication of Israel.
Another sign seen during the march featured the Jewish Star of David wrapped around a Nazi swastika with the slogan: "No British politician should be a 'friend of Israel'."
Sky News has also seen graffiti on the route of the protest which makes a comparison with what is happening in Gaza with what happened during the Holocaust, although it is not known if it was made by anyone on the march.
Incidents of antisemitism were criticised by Prime Minister Rishi Sunak, who also condemned far-right protesters gathered in London on Saturday as Armistice Day was being marked.
Mr Sunak said: "I condemn the violent, wholly unacceptable scenes we have seen today from the EDL and associated groups and Hamas sympathisers attending the National March for Palestine. The despicable actions of a minority of people undermine those who have chosen to express their views peacefully.
"That is true for EDL thugs attacking police officers and trespassing on the Cenotaph, and it is true for those singing antisemitic chants and brandishing pro-Hamas signs and clothing on today's protest. The fear and intimidation the Jewish Community have experienced over the weekend is deplorable.
"All criminality must be met with the full and swift force of the law. That is what I told the Met Police Commissioner on Wednesday, that is what they are accountable for and that is what I expect."
Mayor of London Sadiq Khan said he is in close contact with police, who are investigating claims that pro-Palestinian demonstrators have been targeting Jewish synagogues.
Mr Khan said: "Places of worship are sanctuaries and targeting a synagogue or Jewish worshippers is unacceptable and racist."
It came after videos online appeared to show an individual waving a Palestinian flag and holding a device producing green smoke.
Labour's shadow home secretary Yvette Cooper also denounced both "disgraceful scenes of far-right violence against police officers" and "appalling cases of antisemitic hate, intimidation, and support for terrorist groups like Hamas".
Liberal Democrat home affairs spokesperson Alistair Carmichael added: "The horrific cases of antisemitism and support for terrorist organisations that we have seen on the streets of London today need to be totally condemned. It has no place in our society.
"Those who have participated in this hate and disorder should feel the full force of the law."
A number of the far-right protesters clashed with police, with the Met saying dozens had been arrested.
Later, around 150 pro-Palestinian protesters were detained by police after breaking away from the march through London.
Around 8.30pm, the Met said 126 people had been arrested at that point, but further arrests were to be expected.
Assistant commissioner Matt Twist said in a statement on X (Twitter): "While the Palestine Solidarity Campaign (PSC) march did not see the sort of physical violence carried out by the right-wing, we know that for London's Jewish communities whose fears and concerns we absolutely recognise, the impact of hate crime and in particular antisemitic offences is just as significant.
"At the end of the PSC march, we once again saw breakaway groups behaving in an intimidating manner.
"There were also a number of serious offences identified in relation to hate crime and possible support for proscribed organisations during the protest that we are actively investigating.
"We will soon publish images of some of those we suspect have committed these offences and as we have shown in recent weeks, we will pursue all available lines of enquiry to identify suspects and take action."
More than 1,000 police officers have been drafted in from outside forces to monitor the protests, with the Met saying 1,850 personnel are on duty on Saturday and 1,375 on Sunday.
In a statement ahead of the march, the Met Police said: "We'll be using an extensive set of powers to prevent any disruption whatsoever to remembrance events, policing the demonstration as it passes through parts of the capital, while protecting our communities from those intent on inciting hate, violence and disorder."