More than 100 people have been arrested following violent protests in London on Saturday, the Met Police has said.
Police were attacked by demonstrators, some of whom were far-right activists, after thousands gathered saying they were protecting statues.
PM Boris Johnson said: "Racist thuggery has no place on our streets."
A 28-year-old has been arrested after a man was seen urinating next to a memorial of PC Keith Palmer, who was killed in the 2017 Westminster attack.
Separately on Saturday, a number of peaceful anti-racism protests were held in London and around the country.
Met Police said a man was in custody in Essex after handing himself in after the picture of a man urinating next to the memorial emerged.
Home Secretary Priti Patel said the "desecration" of PC Palmer's memorial was "utterly shameful".
Speaking on Saturday, Met Police Commander Bas Javid added: "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.
"We have immediately launched an investigation, and will gather all the evidence available to us and take appropriate action."
MP Tobias Ellwood, who gave first aid to PC Palmer as he lay dying after being stabbed in the grounds of Parliament by Khalid Masood in 2017, said the image of the man urinating next to the memorial was "abhorrent".
He told the BBC: "He was fully aware of what he was doing, he should step forward and apologise."
Scotland Yard said they had arrested people for offences including violent disorder, assault on police, possession of an offensive weapon, breach of the peace, being drunk and disorderly and possession of Class A drugs.
Prime Minister Boris Johnson tweeted: "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."
Various groups from around the country, including some far-right activists, said they had come to London to protect statues from anti-racism activists.
Hundreds of mostly white men gathered around the Cenotaph war memorial in Whitehall and the statue of Winston Churchill in Parliament Square, which was boxed up to protect it from potential damage after protesters wrote "was a racist" on it last weekend.
As large groups including right-wing protesters moved across London, there were a number of clashes with police in riot gear.
Officers were pelted with missiles, as well as being kicked and punched. Six police officers suffered minor injuries in the clashes.
A statement from the London Ambulance Service said it had treated 15 patients, including two police officers, for injuries at the protests.
"Six of these patients - all members of the public - have now been taken to hospital."
At the scene in Parliament Square
From mid-morning on Saturday, hundreds of men - most of whom were already drinking - headed into Parliament Square.
Some were members of football hooligan networks who had put aside their differences for the day. Others were from the far-right.
But despite claims that the day was about defending British heritage, it soon became violent.
Groups looked for opportunities to attack the police. Bottles and cans were thrown at their lines and horses - smoke bombs and fireworks set off. Journalists who came too close were threatened.
One bare-chested man kicked a police officer to the ground and there were repeated attempts by others to storm barriers as the police contained the trouble.
By late afternoon, some had left to find more beer - only to find their way back blocked by riot police.
Others tried to regroup on nearby streets as the police played cat and mouse - using their numbers to repeatedly contain and block attempts to take violence around the city.
Labour leader Sir Keir Starmer said any violence against police was "completely unacceptable", adding "no ifs, no buts".
He tweeted that the protests in London were "led by those intent on causing violence and sowing hate for their own ends".
The Met Police Federation also described it as "unacceptable", tweeting that its officers "do not come to work to face this level of violence and abuse".
Some anti-racism protests also took place in London and across the UK on Saturday.
Organisers from the Black Lives Matter movement had urged people not to join any rallies planned for the weekend over fears there could be clashes with far-right groups.
Peaceful demonstrations by anti-racism protesters in support of the Black Lives Matter movement took place at Hyde Park and Marble Arch.
In Brighton, thousands formed a mile-long line along the seafront as they staged a silent protest before marching through the city centre.
Demonstrations have been taking place across the world following the death in police custody of African American George Floyd in Minneapolis last month.