A series of explosions killed 28 people and wounded more than 40 at a Shiite political organisation's rally in Iraq, police and medical sources said.

The militant group, Asaib Ahl Haq (League of the Righteous), had been presenting its candidates for elections on April 30 at the rally in eastern Baghdad when three bombs exploded in succession as people were leaving.

A roadside bomb blew up near the main gate, followed by a suicide car bomb after a few minutes and then a final explosion.

A Twitter account, associated with the Al Qaeda offshoot, the Islamic State of Iraq and the Levant (ISIS), praised the attack but did not claim it.

"A knight from the state of Islamic knights has detonated his explosive vest near the Asaib podium while they were parading... killing dozens including commanders," it said.

The attack came as tensions ran high in Iraq before its national election and with Iraqi security forces locked in a four-month battle with ISIL in western Anbar province.

Asaib's main leader, Sheik Qais Khazaali, had just delivered a speech accusing some politicians of aiding terrorism and vowed his movement was ready for any action by ISIS.

"To all ISIS... we are ready. We are prepared," he said.

"We are the defenders of this country. If ISIS is the sickness, were are the medicine."

As people started to leave the stadium, the first bomb exploded and grey smoke rose in the air causing people to scramble for cover.

Shortly afterwards a white mini-van raced to the stadium's main gate and detonated, unleashing a massive ball of fire.

A third and final explosion shook the ground, as Asaib members commandeered cars to rush the injured to hospital.

Asaib, which has formed its own party, Al Sadiqoon (the truth seekers), is accused by some Sunnis and Shiites of carrying out killings and driving families from their homes.

Sheik Khazaali denies the allegations but says his movement defends Iraq against terrorism.

Reuters