LONDON — She organized “Christian patrols,” marched with crosses through heavily Muslim neighborhoods and accosted veiled women to rail against Islam. She was barred from entering any mosque in England. She has been arrested several times, most recently this month, on charges of making threats and abusive remarks.

Still, Jayda Fransen, the 31-year-old deputy leader of Britain First, a fringe group that insists that white Christian civilization is under threat from Muslims, had received relatively little attention.

That changed on Wednesday, when Britain First got unexpected publicity from President Trump, who recirculated three sometimes misleading videos Ms. Fransen had posted on Twitter that purported to show Muslims engaged in violence.

Hours later, Ms. Fransen took to Twitter to thank him. “You’ve shed light on my plight here in Britain, in that I am facing prison for giving a speech in which I criticized Islam,” she wrote. “This is evidence that Britain has become Sharia-compliant,” she added, referring to Islamic law.