The explosions struck as some 10,000 people had gathered at the Industrial Stadium in eastern Baghdad for the Asaib Ahl al-Haq rally, in which the Shiite group planned to announce its candidates for Iraq's parliamentary election on Wednesday.
An Associated Press reporter at the rally heard intense gunfire follow the blasts. Attendees fled to a nearby building under construction in the stadium complex as female parliamentary candidates screamed.
Followers of Asaib Ahl al-Haq carried out deadly attacks against US troops before their withdrawal from Iraq in 2011 and claimed responsibility for the 2007 kidnapping of a British contractor along with his four guards. The group is backed by Iran and says it is sending fighters to Syria.
Its leader, Sheik Qais al-Khazali, spent years in US detention but was released after he was handed over to the Iraqi government. At the rally today, he gave a brief address that challenged the Sunni militants holding two cities in Anbar province.
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The blasts highlight the sectarian violence that's plagued Iraq recently. Last year, the death toll in the country climbed to its highest levels since the worst of the country's sectarian bloodletting between 2006 and 2008.
The United Nations says 8,868 people were killed in 2013, and more than 1,400 people were killed in the first two months of this year alone.
No one immediately claimed responsibility for the attack, though an al-Qaida spin-off group known as the Islamic State of Iraq and the Levant uses similar tactics.