The letter promised retaliation against Yudhoyono for his support of the pending executions of three men convicted of carrying out the 2002 Bali terrorist bombings in which 202 people were killed, the Jakarta Post reported Wednesday.
Yudhoyono spokesman Andi Mallarangeng told reporters that officials would prevent any planned attacks and do what was necessary to track down the persons who posted the threat, which called on Muslims to wage a "war against and kill" Yudhoyono and other Indonesian officials in retaliation for the executions.
"Threats against the president and state officials should not be underestimated. It is terror," Mallarangeng said.
The three Bali bombers -- Amrozi, Imam Samudra and Muklas -- were on death row at the Nusa Kambangan Penitentiary in Central Java, and were scheduled to face firing squads this week, the Post said.
Supporters of the condemned trio gathered Wednesday in the Indonesian hometown of two of the men to protest against their impending execution, SomaliNet reported.