By Laila Bassam

BEIRUT (Reuters) - Lebanese authorities arrested five Syrians and a Palestinian in connection with the twin suicide bombings in central Beirut on Thursday that killed 43 people, a senior security source said on Saturday.

The bombings in a busy residential and commercial area that is a stronghold for Lebanese Shi'ite movement Hezbollah were claimed by Islamic State.

"Within 24 hours the network was arrested in the fastest uncovering of a bombing incident in the country," the source said.

The explosions were the first attacks in more than a year to target a Hezbollah stronghold in Lebanon, as the Iran-backed group steps up its involvement in the war in neighboring Syria.

Lebanon's Hezbollah leader Sayyed Hassan Nasrallah said Syrian and Lebanese detainees were arrested on suspicion of being involved in the bombings, which he squarely blamed on Islamic State. However, he did not say if any of those arrested were Palestinian.

Nasrallah warned his followers of a backlash against ordinary Syrians and Palestinians in the aftermath of the bombings, saying the goal of radical Islamist Sunni fundamentalists, who consider Shi'ites to be heretics, was to sow discord among Sunnis and Shi'ites.

The blasts occurred almost simultaneously late on Thursday and struck a Shi'ite community center and a nearby bakery in the commercial and residential area of Borj al-Barajneh, security sources said. A closely guarded Hezbollah-run hospital is also nearby.

Nasrallah condemned the Paris attacks, saying the region which was reeling under the "earthquake of Daesh barbarity" felt more than any others the pain the French people were now experiencing. Daesh is an Arab acronym for Islamic State.

Islamic State said in a statement posted online by its supporters that its members blew up a bike loaded with explosives in Borj al-Barajneh and that when onlookers gathered, a suicide bomber blew himself up among them. The group said the attacks killed 40 people.

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Hezbollah has sent hundreds of fighters to support Syrian President Bashar al-Assad's forces in the four-year-old conflict over the border.

Government forces backed by Hezbollah and Iranian troops have intensified their fight against mostly Sunni insurgents, including Islamic State, since Russia launched an air campaign in support of Assad on Sept. 30.

(Reporting by Laila Bassam; Writing by Suleiman Al-Khalidi; Editing by Jon Boyle and Digby Lidstone)