A man urinated next to a memorial to PC Keith Palmer, the officer killed in the Westminster terror attack, as far-right protesters ran riot in central London.
The picture - taken amid violent clashes between far-right protesters and police - has sparked outrage and an appeal to identify the man.
It happened as hooligans and far-right groups claimed they were there to protect statues and memorials.
MP Tobias Ellwood, who gave first aid to PC Palmer as he lay dying after being stabbed to death in the grounds Parliament by Islamist terrorist Khalid Masood in 2017, said the image of the man urinating next to the memorial was "abhorrent".
The Tory MP for Bournemouth East and chairman of the Defence Select Committee, tweeted a picture of the man and wrote: "Absolute shame on this man.
Click here to follow our live blog with the latest updates from the violent clashes in central London.
"Of all the images to emerge over these few testing days I find this one of most abhorrent. Please help identify him."
Several hundred demonstrators, mostly white men, attended the protest organised by far-right groups, including Britain First, which claimed they wanted to protect statues such as Winston Churchill from vandalism.
But fights erupted in areas near the Houses of Parliament and Trafalgar Square, as demonstrators repeatedly assailed officers with foul-mouthed chants and missiles, smoke grenades and flares.
Shards of glass were strewn along the streets close to the Cenotaph on Whitehall after bottles were thrown at police officers clad in riot gear.
The violent scenes are in contrast with peaceful demonstrations that took place at Hyde Park and Marble Arch by anti-racism protesters in support of the Black Lives Matter (BLM) movement.
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.
The Metropolitan Police warned people joining demonstrations on Saturday that they must be off the streets by 5pm or risk being arrested.
At around 4pm, the crowd in Parliament Square thinned out after one of the exits was opened, although a few hundred people remained in the area ahead of the 5pm deadline.
The violence has been condemned by the Mayor of London Sadiq Khan and Home Secretary Priti Patel, with the latter branding it as "thoroughly unacceptable thuggery".
Speaking before the clashes, Paul Golding, leader of Britain First, said the crowds had turned out to "guard our monuments".
Mr Golding, who was convicted of a terror offence last month, told the PA news agency: "I am extremely fed up with the way that the authorities have allowed two consecutive weekends of vandalism against our national monuments."
There were similar gatherings on Saturday in Belfast, Glasgow and Bristol with crowds massing around monuments.
In Brighton, more than 1,000 protesters formed a line along the seafront in a Black Lives Matter demonstration.
Protests against police brutality and racism have erupted all over the UK and across the globe following the death of African-American George Floyd at the hands of Minneapolis police nearly three weeks ago.
Last week, the statue of slave trader Edward Colston was pulled down and dumped into Bristol harbour by anti-racism protesters, while the UK's war-time Prime Minister memorial in London was defaced with the words "was a racist".
The UK Protests in support of Black Lives Matter have largely been peaceful, although some have been marred by acts of violence towards police by a comparatively small group of people.
But these attacks on officers and criminal damage on statues have lasted no more than a couple of hours, mostly towards the end of rallies.
