On Sunday about a dozen people were injured, some seriously, when FPI supporters attacked a rally in Jakarta organised by the National Alliance for Freedom of Religion and Faith.
The attacks came after speakers at the rally urged tolerance over the treatment of Ahmadiyya, a minority religious sect that critics say follows a "deviant" form of Islam and should be banned.
Some Muslim groups including the FPI have attacked mosques and buildings associated with Ahmadiyya.
The attack on Sunday's interfaith rally has sparked widespread condemnation and led to calls from other Muslim groups for the government to ban the FPI.
FPI supporters are accused of launching an
unprovoked attack on Sunday's interfaith rally
For years the FPI has had a reputation for attacks on bars and nightclubs in Indonesia during the Muslim fasting month of Ramadan.
But earlier this week the leader of the FPI, Habib Riziek Syihab, was unrepentant, urging supporters to prepare for war.
"We are ready to fight a war against Ahmadiyah with all our followers whenever, wherever," he said.
Susilo Bambang Yudhoyono, Indonesia's president, has called for those behind Sunday's violence to be punished.
He has also called ministers to examine options for banning the FPI under a 1985 law that allows for the dissolution of groups that "disturb public security and peace."
Around 85 percent of Indonesia's 226 million people are Muslims, making it the world's most populous Muslim nation.