FACTBOX: Lebanon's political crisis in outline

(Reuters) - Lebanon's crisis deepened on Wednesday when Hezbollah supporters paralyzed Beirut with road blocks and clashed with government loyalists after the U.S.-backed cabinet took measures against the pro-Iranian groups.

WHO ARE THE OPPOSING SIDES?

Prime Minister Fouad Siniora's government is backed by a coalition of the mainly Sunni Muslim Future group led by Saad al-Hariri, son of assassinated former Prime Minister Rafik al-Hariri; the Christian Lebanese Forces and Phalangist parties; and the Druze Progressive Socialist Party.

The Shi'ite Muslim Hezbollah party is the largest group in the opposition in alliance with the fellow-Shi'ite Amal movement; Christian leader Michel Aoun; and an array of small pro-Syrian parties.

Each side has outside allies. The United States, France and Saudi Arabia are the government's firmest backers. Hezbollah, the only Lebanese faction to keep its arms after the 1975-90 civil war, is supported by neighbouring Syria and Shi'ite Iran.

WHEN DID THEY FALL OUT?

Hezbollah and Amal helped the government coalition to win its parliamentary majority in 2005 elections held just weeks after Syria withdrew its troops from Lebanon. But tensions over the fate of Hezbollah's weaponry sharpened when the group's guerrillas captured two Israeli soldiers on July 12, embroiling Lebanon in a devastating 34-day war with Israel. Five Shi'ite ministers and one Christian resigned from the cabinet in November, 2006, after Siniora rejected demands for a national unity government that would give the opposition veto power. Hezbollah and its allies pressed their campaign with street protests, camping outside Siniora's office in downtown Beirut since December 1, 2006.

WHAT ARE THE UNDERLYING ISSUES?

* The Beirut government on Tuesday attacked Hezbollah over its private communications network and removed the head of airport security in a direct challenge to the group. A cabinet statement described Hezbollah's communications network as "an attack on the sovereignty of the state" and said it would take legal action against anyone involved in it. Hezbollah says the communications network is an integral part of its defense system. It also demands that the airport security chief, a Shi'ite, remain in his post.

* Hezbollah, which showed impressive military prowess in the 2006 war with Israel, wants to keep its weapons for use against the Jewish state. Its adversaries favor Hezbollah's eventual disarmament or integration into the army, citing United Nations Security Council resolutions 1559 and 1701.

* The opposition says the government lost its legitimacy when all the Shi'ite ministers resigned, skewing the sectarian balance in Lebanon's power-sharing system. It demands veto power in any new government and a new electoral law before allowing the election to the presidency of army chief General Michel Suleiman, who both sides have agreed should fill the post. The presidency has been empty since November in the absence of a resolution to the crisis.

* Lebanon's three ruling pillars -- the Christian presidency, the Sunni-led cabinet, and parliament, whose speaker is Nabih Berri, a Shi'ite ally of Hezbollah -- are in disarray.

(Writing by Nadim Ladki in Beirut)