A Syrian schoolboy and his family faced death threats due to an "anti-Muslim message" presented by Tommy Robinson, a court has heard.
The English Defence League founder - whose real name is Stephen Yaxley-Lennon - is being sued by Jamal Hijazi.
Now 17, Jamal was filmed being attacked in the playground at Almondbury School in Huddersfield in October 2018.
Mr Yaxley-Lennon, 38, who accused the teenager of attacking "young English girls", denies libel.
The High Court heard Mr Yaxley-Lennon posted two Facebook videos, viewed by nearly one million people, after footage of the assault on Jamal went viral.
He claimed the boy "was not innocent," had "beat a girl black and blue" and "threatened to stab" another boy at his school, allegations the teenager denies.
Mr Yaxley-Lennon's comments "turned Jamal into the aggressor, and the bully into a righteous white knight", said Catrin Evans QC, representing the Hijazi family.
The allegations had "a devastating effect on Jamal and his family" and led to them being relocated in early 2019, Ms Evans said.
She described Mr Yaxley-Lennon as "a well-known extreme-right advocate with convictions for violence, as well as fraud and drug offences".
He had "used his social media platforms, in particular his Facebook account, to spread his extremist views", Ms Evans added.
She said Mr Yaxley-Lennon "falsely accused Jamal" of attacks "without any direct knowledge of the events in question".
Ms Evans told the court Jamal was bullied shortly after he started at the school in 2016, having come to the UK with his family as refugees from Homs in Syria.
She added: "The bullying, and the failure to do enough about it, culminated in 2018 in a series of incidents in the school, including serious threats made to Jamal."
Ms Evans said a fellow pupil attacked Jamal in October 2018 and "simulated waterboarding" by pouring a bottle of water over his face when he was on the ground.
After the video of the incident went viral, it "received mass national media coverage", Ms Evans told the court.
"The defendant has tried to use Jamal's case as a platform for one of his anti-Muslim rants", Ms Evans added.
Mr Yaxley-Lennon, who is representing himself at the trial, is defending his comments on the basis they are substantially true.
In written submissions, Mr Yaxley-Lennon said he had "uncovered dozens of accounts of aggressive, abusive and deceitful behaviour by the claimant, including acts which speak the truth to the matters complained of".
Referring to the alleged "waterboarding" incident, Mr Yaxley-Lennon added: "Not only was the claimant the bully and not the victim in this incident, he has subsequently been shown to be the aggressor on multiple occasions, indulging in violent and abusive behaviour."
The trial continues.