A suicide bomber blew himself up in a restaurant inside Iraqi parliament building in the Green Zone in central Baghdad on Thursday, killing seven besides himself, including three lawmakers, an Interior Ministry source told Xinhua.

The suicide bomber wearing an explosive vest blew himself up in the restaurant while many members of parliament were having lunch after their ordinary session, the source said.

Three members of the parliament and four employees in the building were killed and 20 others were injured, including five lawmakers, he said.

Earlier, Iraqi state-run television reported that three lawmakers were killed and many others injured, but giving no reason for the blast.

Mahmud al-Mashhadani, the speaker of the parliament, said that the parliament will held an exceptional session tomorrow "as a challenge to terrorism."

The parliament building is located in the heavily-fortified Green Zone. Thursday's blast is an extremely rare security breach in which a bomb was planted and detonated inside the exclusive area because anyone who wants to enter it must go through numerous security scrutiny, including dog sniffing and metal detectors.

The Green Zone has been in the past hit by indirect fires such as mortars, but they were all launched from places outside the enclave.

The attack on one of Iraq's most protected place demonstrated how difficult it is to rein in violence in the war-wracked country despite the ongoing security crackdown involving tens of thousands of joint U.S. and Iraqi soldiers.

Earlier Thursday, an explosive-laden truck detonated on a key bridge over the Tigris River in Baghdad, killing at least five people and injuring 18 others. At least five cars have fallen into the Tigris River below.