Tens of thousands of protesters have joined two rival marches in London - the Unite the Kingdom rally organised by far-right figure Tommy Robinson, and a pro-Palestinian demonstration.
More than 4,000 police officers have been deployed to the capital and are managing a so-called "sterile zone" between the two marches. Officers also used drones, police horses and dogs, and had armoured vehicles on standby.
The Metropolitan Police called it one of the most significant policing operations in years, with tens of thousands of football fans also at Wembley Stadium for the FA Cup Final on Saturday afternoon.
The force said there had been 31 arrests as of 16:30 BST "across the whole operation".
Protesters attending the Unite the Kingdom march gathered in Kingsway, before heading to Whitehall and a rally in Parliament Square.
Many could be seen waving Union flags, with some wearing "Make England Great Again (Mega)" red hats. Chants of "we want Starmer out" could also be heard.
Those gathered told the BBC they had a wide range of views - including some who wanted to see the end of the current government, and others who felt white people, in particular white working class people, were being discriminated against in the UK.
Police had put up barriers to block routes between the two protests.
Alongside Robinson, whose real name is Stephen Yaxley-Lennon, speakers at the rally included former LBC presenter Katie Hopkins, actor-turned-politician Laurence Fox, and TV personality Ant Middleton.
The separate pro-Palestinian march, marking Nakba Day, started in Kensington before heading to Waterloo Place via Piccadilly.
Nakba Day refers to the displacement of Palestinians who fled or were driven from their homes during the war surrounding Israel's creation in 1948-49.
Demonstrators expressed a wide range of views to the BBC, with many describing themselves as both anti-genocide and anti-fascist.
A number of protesters said they were against rising antisemitism in the UK and conscious that some Jewish people reported feeling intimidated by such marches. But they added that Jewish people were welcome to join the demonstration, with many present on Saturday.
Flags and signs reading slogans including "smash the far right" and "Free Palestinian Hostages" could be seen amongst those gathered, many of whom were wearing keffiyehs, seen as a symbol of solidarity with Palestinians.
Speakers at the rally included Your Party co-founder Zarah Sultana, independent MP Diane Abbott and Labour MP Apsana Begum.
The Met said both protests had "proceeded largely without significant incident".
It did not specify the offences nor the event to which all the arrests were connected.
It earlier said two men were arrested near Euston station - one of whom was wanted on suspicion of grievous bodily harm following an incident in Birmingham, and was spotted arriving in London to attend the Unite the Kingdom rally.
The second man was wanted for a separate offence which involved encouraging people to attack a police officer, the force added - having earlier said both men were connected to the incident in Birmingham.
The BBC had witnessed two males being arrested at the pro-Palestinian march by early afternoon.
Ahead of the demonstrations, Met Police Deputy Assistant Commissioner James Harman said the policing operation would cost the force £4.5m.
The Met said the risks meant it had to impose the "highest degree of control", including the first use of live facial recognition cameras as part of a protest policing operation.
Live facial recognition is being used at Euston and King's Cross St Pancras railway stations, where people attending the Unite the Kingdom march were likely to arrive.
There are strict controls on where the marches can go and and what time they must finish.
Drones were used to monitor both protest routes, while officers in Wembley were also set to monitor CCTV feeds from the FA Cup final to identify supporters travelling towards demonstrations.
Eleven foreign "far-right agitators" were blocked from entering the country to attend the Unite the Kingdom event, the government said on Friday.
Among the 11 blocked from entering the UK was the US-based, anti-Islam influencer Valentina Gomez, who attended the first Unite the Kingdom march last September.
In a statement on Friday, Sir Keir Starmer said: "We're in a fight for the soul of this country, and the Unite the Kingdom march this weekend is a stark reminder of exactly what we are up against.
"Its organisers are peddling hatred and division, plain and simple. We will block those coming into the UK who seek to incite hatred and violence.
"For anyone who sets out to wreak havoc on our streets, to intimidate or threaten anyone, you can expect to face the full force of the law."
Ahead of the march, Robinson wrote in a post on X: "Today, we Unite the Kingdom and the West in the greatest patriotic display the world has ever seen.
"The establishment has shown their hands early, and clearly don't want the people united. Tough! It's happening!"
John Rees from Stop the War, one of the organisers of the pro-Palestinian march, said it happens on the same day every year and questioned why the Unite the Kingdom march was allowed to also take place on 16 May.
The police response should have been that it was "unacceptable", he told BBC Radio 4's Today programme.
The Met had previously said it was already in discussion with Unite the Kingdom when the application for the Nakba Day march was made.
Meanwhile, new guidance issued by the Crown Prosecution Service pushes prosecutors to consider whether protest placards, banners and chants viewed on social media may amount to offences of stirring up hatred.
Specialist officers are also primed to take swift decisions to arrest and charge hate speech crimes, which may include arrests for chants referring to "intifada" at the pro-Palestinian march.
Home Secretary Shabana Mahmood said the right to protest was "a cornerstone of our democracy".
She added: "But anyone spreading hate or committing acts of violence will face the full force of the law."
Director of public prosecutions Stephen Parkinson said: "This is not about restricting free speech. It is about preventing hate crime and protecting the public, particularly at a time of heightened tensions."
Lord Mann, the government's independent advisor on antisemitism, told the BBC that organisers of public events should screen contributors to judge the level of risk they pose - and ensure they "stay within the law".
