Tommy Robinson faces jail after being found in contempt of court for filming defendants in a criminal trial and broadcasting the footage on social media.
Mr Robinson, whose real name is Stephen Yaxley-Lennon, could be jailed for up to two years after the ruling by High Court judges.
Disorder broke out outside the court as the verdict filtered through to the crowds of supporters of Mr Robinson.
Police raised their batons after some supporters stormed fences separating them from the media and the public, and beer cans were also thrown at journalists.
An aide to Mr Robinson attempted to diffuse the situation by climbing onto another person’s shoulders and calling for calm.
Howls of fury and disbelief came when the verdict was first announced.
Parts of the crowd began chanting “shame on you” and pointed at the court while others stood in silent disbelief.
A woman with a loud speaker said: “You’re disgusting, the bloody lot of you.”
The former EDL leader broke reporting restrictions to livestream the faces of defendants in a child grooming case in Huddersfield in 2018.
Restrictions were in place to postpone the publication of details of the trial until the end of a series of linked trials.
Tommy Robinson has issued a message to supporters on his Telegram group.
He said: “This is the biggest political stitch up we have ever seen. So every single journalist that photographed Tommy going into this court — are they going to be in the Old Bailey and getting guilty of contempt of court?
“This is the biggest case of ‘one rule for Tommy’ and one for everyone else.
“British justice system and the establishment stinks.”
The 36-year-old, from Luton was initially jailed for 13 months over the illegal broadcast.
The video lasted an hour-and-a-half and was viewed online 250,000 times after being live-streamed on Facebook.
He served two months in jail before being freed after an appeal in August 2018.
But the case was then referred back to the Attorney General, who announced in March that it was in the public interest to bring fresh proceedings against Mr Robinson.
Dame Victoria Sharp, sitting at the Old Bailey with Mr Justice Warby, found Robinson in contempt in three respects.
Robinson, wearing a blue jacket, blue shirt and jeans, showed little reaction as the judge announced the decision.
Dame Victoria said the court will consider what penalty to impose for the contempt, and give full reasons for the decision, at a later date.