Montasser AlDe'emeh runs a centre in Brussels' gritty Molenbeek district -- where several of those who carried out attacks in November lived -- that aims to prevent young Belgians from going to fight in Syria and also help reintegrate those who do so on their return.
Federal prosecutors said he was suspected "of having drawn up and submitted a false affidavit concerning a deradicalistion course taken by Jawad O. At the centre."
The prosecutors said Jawad O., who was not further identified, had been charged with taking part in the activities of a terrorist group.
His brother Khalid O. Was also arrested Monday, they said.
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Flemish TV station VRT reported that Jawad O. Had been detained just as he tried to leave for Syria and that Khalid, an imam at a mosque in the northern city of Malines, had contacted AlDe'emeh at the time to seek his help.
The three men will appear in court within 24 hours to answer the charges, they said.
AlDe'emeh wrote a book about Belgium's jihadi fighters -- believed to number around 500 -- and is a regular contributor to local and foreign media on the issue.