Jailed Islamist cleric Abu Bakar Bashir threatened to wage war if Myanmar continues to harm Muslim Rohingyas, in a letter to the country's president Thein Sein seen on a website Friday.
The 74-year-old is widely regarded as a spiritual leader of radical Islam in Indonesia -- the world's most populous Muslim country -- and is currently serving a 15-year-jail term for funding terror.
"We've heard Muslims screaming in your country because of your acts of evil...you have taken them out from their homes and are killing them," he wrote in the letter dated July 22, which was passed on to followers and published on the website voa-islam.com.
"If you neglect these calls, by Allah our Lord, you have witnessed the fall of proud and conceited countries in the hands of our mujahideen soldiers," he added.
The letter was confirmed as authentic by Son Hadi, the spokesman for Jemaah Anshorut Tauhid (JAT), a group founded by Bashir in 2008 and labelled a terrorist organisation by the United States.
An outspoken supporter of violent jihad, Bashir was convicted in 2010 of financing a teror cell in Aceh province. Earlier this year, the country's top court overturned a lower court's decision to cut his 15-year term.
"You must know that we are brothers as Muslims. Their pains is our pain, their sorrows are our sorrows, and their blood that you shed is our blood too," Bashir wrote. "By the will of Allah, we can destroy you and your people."
Son Hadi said Friday that the letter was submitted on Monday to the Myanmar embassy in Jakarta. The embassy was not reachable for comment.
About 100 Muslim extremists from the Indonesian branch of pro-Caliphate organisation Hizb ut-Tahrir protested Friday outside the Myanmar embassy and vow a Jihad to stop the "Muslim cleansing".
"We are ready to die to help our fellow Muslims in Myanmar. A Jihad is the only way to stop this massacre," one of the protesters on loudspeaker told the crowd, who shouted "Allahu Akbar" (God is Greatest).
Violence erupted in June in Rakhine state, in western Myanmar, between Buddhists and Rohingya, leaving about 80 people dead from both sides, according to official estimates deemed low by rights groups.
Myanmar security forces opened fire on Rohingya Muslims, committed rape and stood by as rival mobs attacked each other during the recent wave of sectarian violence, New York-based Human Rights Watch said Wednesday.
The authorities failed to protect both Muslims and Buddhists and then "unleashed a campaign of violence and mass roundups against the Rohingya", the group said in a report.