Hours after a deadly knife attack in the northern English town of Southport last July, Andrew McIntyre set up a Telegram channel called “Southport Wake Up.” Amid posts riddled with anti-Islamic, antisemitic and anti-immigrant abuse, he urged people to take to the streets, directing them to a mosque and calling for “war.”

Mr. McIntyre, 39, who took part in two of several riots that rattled Britain last summer, was sentenced to seven years in prison last Monday.

He pleaded guilty to encouraging violent disorder and criminal damage, and to possession of a knife. A day after his conviction, Elon Musk, the billionaire tech mogul, bemoaned Mr. McIntyre’s sentencing in a post on his social media platform X, declaring: “Over 7 years prison for social media posts … Whoever gave that sentence deserves prison themselves.”

Mr. Musk’s post, which has been viewed 34 million views times, is just one in a flurry of comments he has made about Britain in recent weeks that have included attacks on the governing Labour Party, and that have increasingly echoed far-right talking points laced with disinformation.

“I think it’s really important to note here just how extreme the individuals that Musk has been defending and engaging with in Europe and in the U.K. in recent months are,” said Joe Mulhall, the director of research for Hope Not Hate, a Britain-based advocacy group that researches extremism, particularly on the far right.