(Adds U.N. security council)

By Mussab Al-Khairalla and Yara Bayoumy

BAGHDAD, April 13 (Reuters) - Leaders from across Iraq's sectarian divide pleaded for unity at a special session of parliament on Friday, gathering under high security to condemn a suicide bombing that tore through the building the day before.

A senior government source said authorities had intelligence that militants were planning an attack on parliament before Thursday's bombing, which killed a member of parliament and wounded two dozen other people in the building's restaurant.

An al Qaeda-backed group, the self-styled Islamic State in Iraq, claimed responsibility in a Web statement for the worst breach of security in Baghdad's most secure area -- the Green Zone that also houses government offices and embassies.

The United Nations Security Council denounced the bombing as a "heinous act of terrorism" and demanded that those using violence against the political process lay down their arms.

Three workers in the cafe had been detained, a top lawmaker from the ruling Shi'ite Alliance bloc said. The Interior Ministry said it would not give details of the investigation.

"We had prior intelligence that there would be an attack on the parliament," the government source told Reuters, without giving specific details of when the information had been received or what the nature of the threat was.

Security was heavy on Friday as parliament met. Vehicles and their drivers were thoroughly searched and mobile checkpoints were set up. Police raided houses inside the sprawling compound.

The bombing came two months into a crackdown in Baghdad that U.S. officials hope will give the government breathing space to pull Iraq back from the brink of civil war between majority Shi'ites and once dominant minority Sunni Arabs.

Scores of lawmakers turned up for the session, including some of those wounded in the attack. Their feet crunched on broken glass littered through the building as they walked to the chamber. One female MP wore a neck brace.

"Whether we are in or out of the government and the political process, we have to find a solution to national reconciliation," Shi'ite Vice President Adel Abdul-Mahdi, who survived an assassination attempt in February, told parliament.

Previous calls for unity by Iraq's leaders have mostly fallen on deaf ears as sectarian violence has spiralled.

"This is undeniably a difficult blow, but it should unify us to confront the evil of terrorism and it proves that terrorism is indiscriminate -- Sunnis, Shi'ites, Kurds and Arabs were maimed in this attack," Deputy Prime Minister Barham Saleh, a Kurd, told Reuters, a message he repeated in parliament.

"THEY ARE A GANG"

Parliament Speaker Mahmoud Mashhadani, a Sunni, opened the special session by asking MPs to read verses of the Koran to mourn the death of Mohammed Awdh, a member of the National Front for Iraqi Dialogue, a small Sunni party.

"We are all in one ship. If this ship sinks we all drown ... We are a people and they are a gang. Have you ever heard of a gang that has conquered a people in history?" he said.

Iraqi officials are investigating how the suicide bomber managed to slip past checkpoints and blow himself up while parliamentarians were eating lunch.

Hasan al-Senaid, a senior lawmaker from the Shi'ite Alliance, said the three cafeteria workers had been detained for questioning but had not been charged. Some parliamentary guards were also being investigated but none were being held.

The senior government source said initial evidence showed a member of a Sunni lawmaker's security team might have played a role in the attack.

The U.S. military had initially said eight people were killed and more than 20 wounded in Thursday's blast. On Friday they revised the toll down to one killed and two dozen wounded, in line with figures from Iraqi officials.

The explosives used in Thursday's attack would have had to pass through an outer checkpoint manned by U.S. and Iraqi troops and inner checkpoints guarded by security contractors and foreign troops in the U.S.-led coalition.

Washington and some Iraqi politicians dismissed suggestions the attack signalled a failure of the U.S.-Iraqi security crackdown in the capital. (Additional reporting by Ahmed Rasheed, Dean Yates and Sami Aboudi in Dubai)

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