Daniel John Allan, 35, carried out the unprovoked attack on a Met Police Sergeant after travelling to London from his home in Sunderland to join the demonstration last month.
He was caught on CCTV and identified thanks to the luminous shorts he had worn for the day, and is due to be sentenced at Southwark crown court today.
When Allan appeared in court in 2017 after a break-in at his former partner’s home, Newcastle crown court heard he had 110 previous convictions on his record.
At least one was a violent offence, and chroniclelive.co.uk revealed how Allan had threatened to smash his ex-partner’s windows and start a fire at the home if she did not speak to him.
During the January 2017 incident, which followed the breakdown of the relationship, Allan drunkenly broke a window and climbed into the home while continuing to make threats.
His latest conviction comes after protests in Whitehall and Trafalgar Square on June 13 which descended into violence with missile thrown at police lines.
Responding to the Black Lives Matters movement and the removal of statues, a counter demonstration led by far right activists was organised with the stated aim of “defending statues”.
Prosecutor Jennifer Gatland told an earlier hearing how Allan had “kicked out” at a police officer from behind in Bridge Street, Westminster, and the attack was carried out “suddenly and without provocation”.
Allan’s lawyer, Eleanor Fraser, said he suffered from depression and anxiety and told the court he had a panic attack when arrested.
“The seriousness of your conduct consists of being part of wider protesters in which missiles were thrown”, said District Judge Nicholas Rimmer told him. “You personally kicked out at (the police sergeant), that appears to have catalysed the violence.”
Detective Constable Steve Nicholls, from Scotland Yard, called the attack “shameful and totally unacceptable”.
Allan, from Sunderland, pleaded guilty to violent disorder.