Brussels (AFP) - Belgian prosecutors said Monday they have arrested a high-profile expert on jihadi fighters and charged him with providing a suspected extremist with a false affidavit that he was on a deradicalisation course.

Montasser AlDe'emeh runs a centre in Brussels' gritty Molenbeek district -- where several of those who carried out November's Paris attacks 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 charged with "having drawn up and submitted a false affidavit concerning a deradicalisation course taken by Jawad O. at the centre."

He was conditionally released after being charged.

Prosecutors said Jawad O. was arrested on November 24, a few days after jihadists killed 130 people in a series of attacks across Paris which increasingly appear to have been coordinated from Brussels.

The prosecutors said Jawad O. -- who has not been further identified -- had been charged with taking part in the activities of a terrorist group. They gave no further details.

His brother Khalid O. was also arrested and released on Monday.

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 Belgian city of Malines, had contacted AlDe'emeh at the time to seek his help.

The prosecutors said AlDe'emeh and Khalid O. submitted the false affidavit to a court hearing on November 27 in the hope of obtaining Jawad's release.

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.

The subject is hugely controversial in Belgium, which in proportion to its size is Europe's largest source of foreign fighters heading to Syria and Iraq, with the government fearing they will return home radicalised and battle-hardened.