* Beirut blast targeting Iranian centre caused by two suicide bombers
* Rush hour attack kills six, including bombers
* Radical Sunni group claims responsibility for attack
* Attack follows arrest of leading al Qaeda-linked militant (Adds reaction)
By Laila Bassam and Alexander Dziadosz
BEIRUT, Feb 19 (Reuters) - Two suicide bombers targeted the Iranian cultural centre in Beirut on Wednesday, killing four people and themselves in an attack claimed by Sunni militants who said it was a response to the intervention of Iran and Hezbollah in the Syrian war.
The army said two cars packed with explosives had been used in the rush hour attack in the predominantly Shi'ite southern suburbs of Beirut. Similar tactics were used in a twin suicide attack on the nearby Iranian embassy in November.
The Abdullah Azzam Brigades, an al Qaeda-linked group, claimed responsibility for the attack, which wounded more than 100 people and was condemned by Lebanon's Sunni Prime Minister Tammam Salam as an act of terrorism.
In a post on Twitter, the Brigades described the "twin martyrdom operation" on the Iranian centre as retaliation for Hezbollah's role in Syria and pledged more attacks. The blast went off about 20 metres (yards) from the targeted building.
The judge investigating the attack said a total of 160 kg of explosives had been used. The Iranian ambassador in Lebanon said the dead included a Lebanese policeman who had been guarding the cultural centre, but none of its staff were wounded.
The area is a stronghold of the Shi'ite movement Hezbollah, which is fighting alongside President Bashar al-Assad's forces in the civil war in neighbouring Syria. It was the seventh such bombing in Beirut's southern suburbs since July.
The war in Syria has exacerbated Sunni-Shi'ite tensions in Lebanon, triggering violence including frequent clashes between armed groups in the northern city of Tripoli.
The attack follows a new drive by Lebanese leaders to stem the impact of the Syria conflict on their country.
Rival politicians finally agreed on a new government on Saturday after Lebanon went for 11 months without a cabinet, while the security forces last week captured a man described as the al Qaeda-linked mastermind of recent attacks and seized a number of cars rigged with explosives and ready for use.
IRAN THE TARGET
Wednesday's attack using a Mercedes and a BMW blew out the windows of a nearby orphanage run by a Sunni charity. Children were peering out and screaming "bomb, bomb." Some were crying.
A man working at a sweet shop opposite the bomb site said the blast shook the entire area. "We heard one explosion and then another," he said.
Human remains were found nearby. The casualties included a number of children.
Radical Sunni Islamists sympathetic to the anti-Assad rebellion have pledged to attack Hezbollah on Lebanese soil for helping Assad, a member of the Alawite offshoot of Shi'ite Islam who is also backed by Shi'ite Islamist Iran.
"The attack on the Iranian cultural centre is a response to the fighting by the Party of Iran (Hezbollah) side by side with the criminal regime in Syria," the Abdullah Azzam Brigades said in their statement.
"We will continue, with God's strength, to target Iran and its party in Lebanon ... to achieve our two just demands: One, getting the fighters of the Party of Iran out of Syria; Two: releasing our prisoners from the oppressive Lebanese prisons."
Hezbollah leader Sayyed Nasrallah had said on Sunday his group would continue to fight in Syria, describing Hezbollah's involvement there as necessary to stop the spread of radical Sunni militancy.
"Hezbollah will not withdraw from a battle it deems necessary on a strategic level," Ali Ammar, a Hezbollah MP, told journalists at the scene of the blast.
The Future Movement of heavyweight Sunni politician Saad al-Hariri condemned the attack but called on Hezbollah to pull out of Syria so as to deny the militants any excuse for attacks.
Hariri, who is backed by Saudi Arabia, vowed last week to fight radicalism in the Sunni community and distanced his movement from the radical Islamist groups fighting Assad.
Hezbollah accused Saudi Arabia of carrying out the November attack on the Iranian embassy, which was also claimed by the Abdullah Azzam Brigades. Sunni power Saudi Arabia backs both Hariri and the Syrian opposition.
Numerous Lebanese politicians live in the area of Wednesday's blast, not far from Beirut airport. The Kuwaiti embassy and a Lebanese army barracks are also nearby.
Soldiers secured the area as fire trucks and ambulances raced to the scene. The charred and twisted remains of a number of cars littered the site, one flipped onto its roof.
A wounded man was carried away on a stretcher and a young girl was evacuated by two men. Glass covered the road and the facades of nearby buildings were torn off.
Prime Minister Salam condemned the attack.
"Amid the positive atmosphere that followed the formation of the government, terrorism has struck a new blow to Lebanon through this bombing ... which underlines the determination of the forces of evil to hurt Lebanon and its citizens and sow discord," Salam said in a statement. (Additonal reporting Tom Perry, Erika Solomon, Oliver Holmes, Stephen Kalin and Reuters TV; Writing by Tom Perry; Editing by Samia Nakhoul and Jon Boyle)