Supporters of Tommy Robinson threw cans and bottles at police outside the Old Bailey after the EDL founder was jailed for contempt of court.
Robinson, real name Stephen Yaxley-Lennon, was jailed for nine months after he risked prejudice to a sex abuse trial by posting a video of defendants on Facebook.
He will spend about nine weeks in prison due to time he has already served.
Outside the Central Criminal Court his supporters booed as news of the sentence filtered out.
The crowd marched towards the building and chanted, “We want Tommy out” before some began hurling drink bottles and cans at police.
Officers put on helmets as violent clashes erupted.
Video footage showed Robinson’s backers letting off air horns and waving flags as they reacted to the sentence in the cramped street.
Some 20 minutes later, the trouble appeared to have died down. Police in riot gear have returned to their vans.
Inside, Robinson showed little emotion as he was led down to the cells after a judge told him he had lied to the court.
Dame Victoria Sharp said: “The respondent cannot be given credit for pleading guilty. He has lied about a number of matters and sought to portray himself as the victim of unfairness and oppression.
“This does not increase his sentence, but it does mean that there can be no reduction for an admission of guilt.”
Richard Furlong, defending, raised the possibility of an appeal against the court’s decision on contempt.
Additional reporting by PA
