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”.
Robinson, whose real name is Stephen Yaxley-Lennon, called the sentence an “absolute joke”.
In his Telegram account, Robinson said: “Sentenced to prison for journalism. Time for protests to start.”
Outside court, Robinson’s supporters 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.