Grim-faced leaders entered the conference room at a hotel in Doha separately for talks chaired by Qatari Prime Minister Sheikh Hamad bin Jassim bin Jabr al-Thani.
At a brief opening session on Friday night, Qatari Emir Sheikh Hamad bin Khalifa al-Thani urged government and opposition leaders to reach an agreement to end a political standoff that has paralyzed the government for 18 months and left Lebanon without a president since November.
"It is only natural that tension prevails early in the talks," one delegate said. "But the ice will be broken as the talks go on."
On Thursday, Arab mediators reached a deal to end Lebanon's worst internal fighting in nearly two decades and create a framework for the talks hosted by Qatar.
The clashes killed 81 people and exacerbated sectarian tensions between Shi'ites loyal to the Iranian-backed Hezbollah group and Druze and Sunni followers of the U.S.-supported ruling coalition.
Distrust between the two sides has been running high for months and diplomats said the main challenge for the hosts was to rebuild confidence between the rivals who have some major obstacles to overcome.
"Chances of success and failure are 50-50. It is a very complex crisis and the hurdles are so big that it would require a huge effort to resolve," a Lebanese politician said.
Sheikh Hamad shuttled between the leaders on Friday night to get them to agree on an agenda and soften their positions.
The leaders are scheduled to discuss power sharing in a new government and the basis of an election law but the ruling coalition will demand that Hezbollah's weapons and ties with the state will be the first item on the agenda after the anti-Israel group turned its arms against its political rivals.
NO DEADLINE SET
There has been no deadline set for the talks but diplomats said if no understandings were reached over the next two to three days, then an agreement might prove elusive.
"The issue is not simple, " government minister Ahmad Fatfat said. "Everyone will work day and night to reach a solution."
Washington blames Syria and Iran for Hezbollah's brief seizure of parts of Beirut last week which forced the government to rescind two decisions that had triggered the escalation.
The opposition has demanded more say in a cabinet controlled by factions opposed to Syrian influence in Lebanon.
Syria, which backs the opposition and is an ally of Iran, said it supported the Qatari-led Arab League initiative.
The anti-Damascus factions have long accused the opposition of seeking to restore Syrian domination that was ended in 2005 when Syria -- facing international pressure and Lebanese protests following the killing of former Prime Minister Rafik al-Hariri -- ended a 29-year military presence.
Saudi Arabia, a strong backer of the ruling coalition, also stated its support for the deal. Riyadh said this week that Hezbollah's campaign could affect Iran's ties with Arab states.
The ruling coalition's refusal to yield to the opposition's demand for veto power in cabinet triggered the resignation of all its Shi'ite ministers in November 2006. Lebanon was plunged into its worst political crisis since the civil war.
A deal would lead to the election of army commander General Michel Suleiman as president. Both sides have long accepted his nomination for a post reserved for a Maronite Christian in Lebanon's sectarian power-sharing system.