LONDON — The leaders of a British far-right group, which had gained notoriety after President Trump recirculated unverified anti-Muslim videos it had posted on social media, were arrested on Thursday.

Paul Golding, the leader of Britain First, was detained in Belfast, Northern Ireland, the group said, where he was accompanying his deputy, Jayda Fransen, to her court hearing on earlier charges related to using “threatening, abusive, insulting words or behavior” during an anti-Islam speech in August that prosecutors said could qualify as incitement to racial hatred. She has denied the charges.

Shortly after her court appearance, British news media said she was arrested again, this time as part of a police investigation into “an incident at a peace wall” in Belfast on Wednesday.

Earlier, the Police Service of Northern Ireland said on Twitter that detectives investigating speeches made at the Northern Ireland Against Terrorism Rally on Aug. 6 “have arrested a 35-year-old man in the Belfast area today.” The post did not identify Mr. Golding or the offense.