A suicide attack at a meeting between Iran's elite Revolutionary Guards and tribal chiefs killed at least 49 people in the city of Pisheen, near the Pakistan border, on Sunday.
Several tribal leaders and seven members of the elite Revolutionary Guards died when the attacker, who was wearing an explosive belt, set it off at the meeting.
Iranian officials warned that the death toll could rise further.
President Mahmoud Ahmadinejad claimed that the attack had been plotted in Pakistan and Iran's Foreign Ministry summoned the neighbouring country's charge d'affaires over the attack.
Pisheen is located in the Sistan-Baluchestan province, which has witnessed rising Sunni insurgency against the government controlled by the Shiites.
A senior official said that Abdolmalek Rigi, the chief of an underground Sunni rebel group called Jundallah (Soldiers of God), had claimed responsibility for the attack.
The Jundallah has often targeted the Revolutionary Guards, members of an elite force which safeguards the regime from external and internal dangers.
Ahmadinejad accused Pakistan of sheltering Jundallah members and urged the government to take action against them immediately.
Ali Larijani, the speaker of the Iranian parliament, blamed the United States for the attack, saying, "This is the sign of America's animosity against our country".