Lebanese troops blocked streets in Beirut with tanks and barbed wire for several hours on Sunday after the killing of a protester outside the Iranian embassy raised factional tensions already inflamed by the war in Syria.
The man died during a clash between rival groups of Shi'ite Muslims after militiamen from the Iranian-backed Hezbollah movement opened fire when protesters drew up at the embassy.
The Lebanese army, which has limited means to impose itself on the many armed factions still active two decades after Lebanon's own civil war, deployed armored vehicles and set up roadblocks to cordon off the city center and neighborhoods controlled by Hezbollah. Traffic was restored toward evening.
Demonstrators from a variety of groups, including Sunni and Shi'ite Muslims and Christians, marched in the city center in protest at Hezbollah's newly prominent role in supporting Syrian President Bashar al-Assad - its fighters helped Assad's troops retake the strategic border town of Qusair last week.
When protesters from a small Shi'ite party opposed to Hezbollah arrived at the Iranian embassy south of the city center in a bus, a Reuters journalist saw them clash with black-clad Hezbollah militiamen, who opened fire.
Lebanese security officials said one of the protesters, who was unarmed, was killed and several people were hurt.
Fear has increased of troubles in Syria - which long dominated its smaller neighbor - disrupting Lebanon's fragile balance since Hezbollah leader Sheikh Hassan Nasrallah vowed two weeks ago to fight for Assad until a final victory.
Beirut has been rebuilt after 15 years of civil war that ended in 1990, but security has proven elusive, jeopardizing the hope of restoring its prosperity.
Hotel owner Ali Hammoud said Sunday's violence risked deterring tourists: "What happened today makes us feel there is a very difficult period ahead. We are bringing disasters upon ourselves by interfering in others' affairs ... No one will come to Lebanon now; our concern now is just to stay alive."
A member of the Lebanese parliament who supports the Sunni former prime minister Saad Hariri said Hezbollah must pull its fighters out. "Hezbollah has driven Lebanon into a tunnel without end," said Nuhad Mashnouq. "There will be an inevitable calamity in Lebanon ... They must withdraw from Syria."
Violence within Lebanon's Shi'ite community is not unknown but is less visible than that between Lebanese Shi'ites and Sunnis since the two-year-old Syrian civil war veered onto to a clearly sectarian course, pitting Assad and his fellow Alawites, an offshoot of Shi'ite Islam, against mainly Sunni rebels.
That division has inflamed a wider confrontation across the Middle East between Shi'ites supported by non-Arab Iran and the Sunnis who rule most Arab nations. Western powers and Turkey have also rallied behind the rebels, despite misgivings over Islamist radicals in their ranks, while Russia has armed Assad.
The United States and Russia have been trying to bridge differences by pushing the warring sides to peace talks in Geneva, but divisions among the opposition and wrangling among the major powers over whether Assad must step down, has left the prospects for the conference unclear.
Britain and France have broken ranks with other European powers and the United States to say they may join Saudi Arabia and Qatar in arming the rebels as a way to end a war that has killed over 80,000 people. British Foreign Secretary William Hague said on Sunday London was still "very reluctant" to do so.
"People have understandable concerns about the idea of sending arms to anybody in Syria," he said. "On the other hand, at the moment, people are being killed in huge numbers while the world denies them the right to defend themselves."
Israel, which has bombed what it suggests were Iranian missiles in Syria headed for Hezbollah, said on Sunday it aimed to stay out of the war, though it is concerned by fighting on the U.N.-policed ceasefire line in the Golan Heights that might bring Islamists toward its border.
"Israel is not getting involved in the civil war in Syria, as long as the fire is not directed at us," Prime Minister Benjamin Netanyahu told his cabinet in broadcast remarks.
In Syria on Sunday, opposition activists reported fighting north of Aleppo, the biggest city and commercial hub, and said rebel forces in the area were bracing for an offensive by Assad's forces following their success at Qusair, near Homs.
To the east on Aleppo, the Syrian air force bombed the rebel-held town of Raqqa, activists said, while rebels were trying to capture a major military base nearby.