Tommy Robinson has been jailed for nine months over contempt of court.
The ex-English Defence League leader was found guilty last week of interfering with the trial of a sexual grooming gang at Leeds Crown Court in May 2018.
Two Old Bailey judges said his Facebook Live video of defendants in the trial had encouraged “vigilante action”.
A social media account in Robinson’s name called the sentence an “absolute joke” and said it was time to protest.
Outside the London court, supporters of Robinson, whose real name is Stephen Yaxley-Lennon, booed as news of his sentence reached them.
The crowd marched towards the Old Bailey chanting “we want Tommy out” before some began pelting police with bottles and cans.
Some supporters then moved to College Green, outside Parliament, where the Press Association reported journalists were verbally abused, physically intimidated and had their equipment attacked before police officers arrived.
During the 2018 case at Leeds Crown Court, reporting restrictions had been put in place postponing the publication of any details until the end of a series of linked trials involving 29 defendants.
However, Robinson, 36, from Luton, broadcast footage from outside the court on 25 May 2018, while the jury in the second trial of the series was considering its verdict.
The video lasted an hour-and-a-half and was viewed online 250,000 times, after being live-streamed on Facebook.
In a written ruling, Dame Victoria said Robinson had claimed his intention in making the broadcast was to “denounce the media” for their behaviour.
But the judges found he had encouraged others “to harass a defendant by finding him, knocking on his door, following him, and watching him”.
This created “a real risk that the course of justice would be seriously impeded”, she said.
Robinson was originally jailed for 13 months on the day of the Facebook broadcast, but was released two months into his sentence after winning an appeal.
The case was then referred back to Attorney General Geoffrey Cox, who announced, in March this year, that it was in the public interest to bring fresh proceedings.
The nine month jail sentence served on Thursday includes six months for the Leeds Crown Court offence last year and another three months for contempt of court, following a suspended sentence given at Canterbury Crown Court in May 2017.
At the Old Bailey on Thursday, Dame Victoria Sharp told Robinson that the time he previously spent behind bars for contempt would be taken into account, reducing his sentence to 19 weeks – of which he would serve half before being released.
Robinson’s barrister Richard Furlong raised the possibility of an appeal against the court’s decision and was told he has 28 days to apply.
The attorney general said the sentencing illustrated how seriously the courts would take matters of contempt and urged everyone to think carefully about whether their social media posts could amount to contempt of court.
What is contempt of court?
Contempt of court laws exist to ensure people get fair trials. The idea is that juries must not be influenced by anything but the evidence they hear in court.
The rules apply to everyone from journalists to people posting comments on social media, and even jurors.
If someone interferes with a trial, the defendants can walk free and a new trial may have to be held.
The maximum sentence for contempt of court is two years in prison, but it can also be punished with an unlimited fine.
Contempt includes publishing anything that creates a substantial risk of seriously prejudicing “active” criminal proceedings. Proceedings become “active” when a suspect is arrested.
Someone could also be in contempt by actions including taking photographs or film, recording what is said in court or talking to a jury member about a case.