A man is being held by police after a picture on social media showed him appearing to urinate on a memorial.
A statement from the Metropolitan Police said: “A 28-year-old man has been arrested on suspicion of outraging public decency after a man was photographed apparently urinating on the memorial dedicated to PC Keith Palmer.
“The incident is believed to have taken place in the afternoon of Saturday, June 13.
“The man is currently in custody in Essex after presenting himself at a police station.”
In an earlier statement about the picture, Metropolitan Police Commander Bas Javid said: “We are aware of a disgusting and abhorrent image circulating on social media of a man appearing to urinate on a memorial to PC Palmer.
“I feel for PC Palmer’s family, friends, and colleagues.
“We have immediately launched an investigation, and will gather all the evidence available to us and take appropriate action.”
PC Palmer was stabbed to death in the 2017 terror attack in Westminster.
Home Secretary Priti Patel last night described behaviour at the London protests as “extreme thuggery”.
She added: “We have seen some shameful scenes today, including the desecration of PC Keith Palmer’s memorial in Parliament, in Westminster Square, and quite frankly that is shameful, that is absolutely appalling and shameful.”
Labour leader Sir Keir Starmer said the violence against police had been “completely unacceptable”.
He tweeted: “Today’s (Saturday) protests in London were led by those intent on causing violence and sowing hate for their own ends. We must not let them win.”
MP Tobias Ellwood, who gave first aid to PC Palmer as he lay dying after being stabbed to death in the grounds of Parliament by Khalid Masood in 2017, said the image of the man urinating next to the memorial was “abhorrent”.
When asked whether he thought the man may not have noticed the memorial, Mr Ellwood said: “I don’t agree with that at all.
“This is the gates of Westminster, the most iconic symbol of democracy in the world next to a monument for PC Keith Palmer.”
More than 100 people were arrested at a protest in London, which was condemned by Prime Minister Boris Johnson as “racist thuggery”.
Six police officers suffered minor injuries in violent clashes as several hundred demonstrators, mostly white men, attended the protest organised by far-right groups which claimed they wanted to protect statues such as Winston Churchill from vandalism.
But the demonstration turned violent after hundreds of self-proclaimed “statue defenders” took over areas near the Houses of Parliament and Trafalgar Square and hurled missiles, smoke grenades, glass bottles and flares at police officers.
Many of those present were drinking, and there were a number of clashes with police in riot gear as crowds chanting “Tommy Robinson” and “England” while raising their arms surged towards lines of officers.
Outbursts of violence continued around the city after the 5pm deadline had passed, with the last few protesters removed from Parliament Square at around 6.45pm.
By 9pm, more than 100 people were arrested during the protest for offences including breach of the peace, violent disorder, assault on officers, possession of an offensive weapon, possession of class A drugs, and drunk and disorder.
Prime Minister Boris Johnson slammed the “racist thuggery” witnessed at the protests.
He wrote on Twitter: “Racist thuggery has no place on our streets. Anyone attacking the police will be met with full force of the law.
“These marches and protests have been subverted by violence and breach current guidelines.
“Racism has no part in the UK and we must work together to make that a reality.”
London Ambulance Service said it had treated 15 people for injuries, including two police officers.
Six of these patients, all members of the public, had to be taken to hospital.
Peaceful demonstrations took place at Hyde Park and Marble Arch by anti-racism protesters in support of the Black Lives Matter (BLM) movement.
While the official BLM protest had been called off over fears of conflict, there were reports of some skirmishes between anti-racism protesters and members of far-right groups outside Waterloo Station.
Mayor of London Sadiq Khan condemned the brutality aimed at police, writing on Twitter: “Millions of Londoners will have been disgusted by the shameful scenes of violence, desecration and racism displayed by the right-wing extremists who gathered in our city today.”
On Friday, statues in Parliament Square – including of Winston Churchill, Nelson Mandela and Mahatma Gandhi – were boarded up to prevent them being targeted by protesters both from the Black Lives Matter movement and far-right groups.
There were similar gatherings on Saturday in Belfast, Glasgow and Bristol with crowds massing around monuments.
Avon and Somerset Police said around 300 people attended the demonstration in Bristol city centre, with two arrests made.
Meanwhile, in Brighton, more than 10,000 protesters attended a Black Lives Matter event in the city centre.
A Sussex Police spokesman said the demonstration was peaceful and no arrests were made.