At least 16 people have been killed and 45 others wounded after gunmen exploded a series of bombs in rapid succession across Baghdad, the interior ministry has said.
Security officials said that at least five bombs rocked Iraq's central bank in the capital within one hour on Sunday, as employees were leaving.
The first bomb blast caused a local electricity substation to explode, which in turn led black smoke to rise over the surrounding neighbourhood.
The gunmen were continuing to occupy the building and the attackers had posted snipers on the roof of the bank in an attempt to deter police and soldiers from wrestling back control, according to the defence official.
Major General Qassim Atta, a spokesman for the security forces in Baghdad, said soldiers and police were "besieging" the attackers whom he described as "a terrorist group."
He said it was unclear if they had intended to rob the bank, target its employees and take hostages, or destroy the building.
The bomb blasts came one day before the opening of the second parliament since the US-led invasion that ousted Saddam Hussein in 2003.
The opening session of the Council of Representatives marks one of the few tangible forward steps taken by Iraq's politicians since a general election on March 7 resulted in deadlock between rival parties.
Diplomats and politicians, warned ahead of Monday's opening that a new government continues to appear some way off, and that it may be several months before the fine detail on the country's new leaders takes shape.
The Iraqiya bloc, led by former prime minister Iyad Allawi, won most seats, 91, in the election, followed closely by prime minister Nouri al-Maliki's State of Law Alliance, which won 89, but both have failed to build a coalition government.
In a sign that the political tempo may be speeding up, Allawi and Maliki held a long-awaited meeting on Saturday, which was described as "friendly and positive," according to a brief statement released by the prime minister.