Police say they are questioning 57 members of a hardline Muslim group following a bloody attack on an interfaith rally in the Indonesian capital, Jakarta.
Fearing violence during the arrests, some 1,500 troops were deployed at the headquarters of the Islamic Defenders Front.
The group, known as FPI, has a long record of torching buildings of rival groups, vandalising nightspots seen as un-Islamic and hurling stones at Western embassies.
National police spokesman Abubakar Nataprawira said some of those detained were suspects in a weekend attack on a rally attended by hundreds of Christians, moderate Muslims and members of the minority sect, Ahmadiyah.
At least 12 people were injured in Sunday’s violence, four of them seriously.