Jakarta. A hardline Muslim leader arrested ahead of Friday's anti-Ahok rally in Jakarta will be kept in custody for at least 20 days to be interrogated over an alleged plot against the government, police said on Saturday (01/04).

Muhammad Al Khaththath and four other men were arrested just hours before Muslims took to the streets of Jakarta on Friday morning in yet another rally against Jakarta Governor Basuki "Ahok" Tjahaja Purnama, who is a Christian of Chinese descent.

Police subsequently took Al Kaththtath, the secretary general of the Indonesian Muslim Forum (FUI), to their Mobile Brigade headquarters in Depok, West Java, for questioning.

Police said they decided on Saturday morning to detain Al Khaththath, who will remain in custody for at least 20 days to prevent him from fleeing or destroying evidence.

"We detained the suspect on objective grounds," Jakarta Police spokesman Chief Comr. Argo Yuwono told the Jakarta Globe.

Thousands of white-clad protesters flocked to Jakarta's Istiqlal grand mosque on Friday morning before marching to the Presidential Palace later in the afternoon.

Muslims have been demanding that Jokowi dismiss Ahok, currently on trial on blasphemy charges after he made a comment on a Koranic verse during a speech in September last year.

Friday's anti-Ahok rally was the fifth in the last six months. The protests have stoked anti-Christian and anti-Chinese sentiments which observers say could scupper Ahok's bid for re-election on April 19.

The previous rallies were mostly led by the hardline Islamic Defenders Front (FPI). Several of its leaders are currently also under police investigation, having been named suspects over a range of allegations.