Suicide bombers killed at least 31 people Friday at a sports stadium hosting a campaign rally for thousands of supporters of a militant Shiite group before parliamentary elections, authorities said — an attack that could unleash more sectarian violence.

A breakaway al-Qaeda group, the Islamic State of Iraq and Syria (ISIS), asserted responsibility for the attack at the Industrial Stadium in eastern Baghdad, where about 10,000 supporters of the Iranian-backed Asaib Ahl al-Haq group had gathered.

ISIS said on a militant Web site that the bombings were to avenge what it called the killing of Sunnis and their forced removal from their homes by Shiite militias. The authenticity of the claim could not be independently verified.

The bombings at the heavily guarded stadium struck about 10 minutes apart, according to two Associated Press reporters at the rally. Intense gunfire rang out after the first blast and continued throughout the assault, but it is not uncommon for Iraqi security forces to fire into the air after major attacks.

As female parliamentary candidates screamed and prayed for safety, some in the crowd fled to a nearby building under construction in the complex. Others ran from the stadium or took refuge behind the large stage erected for the rally.

Adding to the panic was the appearance overhead of a low-
flying small aircraft that dropped election pamphlets.

An AP driver outside the stadium’s main gate said he was thrown back by the first blast. He said guards around him began firing in all directions.

The rally was addressed by Asaib Ahl al-Haq’s leader, Qais al-Khazali, a young cleric who had spent years in U.S. detention but was released after he was handed over to the Iraqi government. In his speech, he challenged the Sunni militants holding parts of two cities in Anbar province, which is predominantly Sunni.

“We are ready and prepared to defend this nation,” said Khazali, a onetime close aide of anti-U.S. Shiite cleric Moqtada al-Sadr. “Let it be known that Asaib will be the remedy.”