LONDON — The authorities in Belgium announced the arrests of six men on Thursday, and a court prolonged the detention of two others, in connection with a plot to attack the capital, Brussels, on New Year’s Eve.

The six men were arrested after searches of seven houses in the Brussels area, amid growing scrutiny of a motorcycle club called the Kamikaze Riders. By Thursday evening, three of the six had been released after questioning, while the other three were being held for at least an additional 24 hours.

“There are persons in the group who have been linked to Sharia4Belgium,” a radical Salafist organization that the Belgian government has designated as a terrorist group, a spokesman for the federal prosecutor’s office in Brussels said.

The group’s Facebook page suggests that the members, all Muslims from the Brussels region, attend motor sports events, but it makes no mention of radicalization, extremism or violence. However, one of the men, Said Abu Shahid, has a YouTube account with videos supporting the Islamic State.