The leaders of Syria and Saudi Arabia arrived in Beirut on Friday for an unprecedented effort to avert a simmering crisis in Lebanon.
Tensions are mounting over expected indictments in the 2005 assassination of former Lebanese Prime Minister Rafik Hariri.
Lebanese Prime Minister Saad Hariri, the son of the slain statesman, and President Michel Suleiman were at the airport as Saudi King Abdullah and Syrian President Bashar Assad landed.
It was a strong public show of cooperation between Saudi Arabia and Syria, which for years vied for influence over Lebanon.
Many fear that new clashes between Lebanon's Shiite and Sunni communities could break out if the international tribunal investigating Hariri's death implicates Lebanon's Shiite militant group Hezbollah, which is Syria's main ally in Lebanon.
Hezbollah militants swept through Sunni neighbourhoods of Beirut in 2008, after the government moved to curb the group's military communications network.
Hariri was a Sunni leader with strong links to Saudi Arabia.
The international tribunal investigating Hariri's death has not announced who will be charged, but the leader of Hezbollah said last week members of his group will be among those indicted.
No details were released about Friday's one-day summit, which marks the first time Syrian President Bashar Assad has visited Lebanon since Hariri's killing in a truck bombing on Valentine's Day 2005. Many in Lebanon blame Syria for the killing, charges that Damascus denies.
Security was tight throughout Beirut on Friday as helicopters buzzed overhead.
Hariri's death prompted the rise of a U.S.- and Saudi-backed coalition known as March 14, named after the day of massive anti-Syrian protests in 2005 dubbed the "Cedar Revolution."
The demonstrations eventually led to the withdrawal of Syrian troops, ending almost three decades of Syrian domination that was established during Lebanon's civil war.
Hezbollah leader Hassan Nasrallah's announcement that his militia members would be implicated in Hariri's slaying appeared to be an attempt to undercut the effects of any indictment, and he dismissed the international tribunal as an "Israeli plot."
Regional tensions also are high over recent reports that Syria sent Scud missiles to Hezbollah and suspicions that Hezbollah patron Iran wants to acquire nuclear weapons. Syria denies sending Scuds.