A protester was fatally shot in clashes outside the Iranian Embassy in Beirut on Sunday as Lebanon struggles to contain the backlash from the Shiite Hezbollah movement’s military support for President Bashar al-Assad in Syria.

About 30 demonstrators had gathered for a sit-in to protest Iran-backed Hezbollah’s involvement in Syria when they were attacked by a group of Hezbollah supporters brandishing sticks, witnesses said. The Lebanese army confirmed that gunfire had broken out, killing one of the demonstrators, but it was unclear whether those shots were fired by the assailants or embassy guards.

There has been a surge in security incidents in Lebanon since Hasan Nasrallah, the leader of the Shiite group, announced his full backing for Assad in late May. The announcement added to fears that the Syrian civil war, which has increasingly sectarian undertones, will inflame tensions in precariously balanced Lebanon.

The largely Sunni Free Syrian Army frequently fires rockets on Shiite areas of the Bekaa Valley and last week saw the first serious clashes between Hezbollah and Syrian rebels on Lebanese soil. That followed the Syrian government’s capture of the border town of Qusair on Wednesday with backing from the movement’s guerrilla fighters.

Analysts have warned that the increasingly visible role of Hezbollah, a formidable political and military force in Lebanon, is likely to stir discontent not only between Sunnis and Shiites in the country but also within the Shiite community.

Gen. Hassan Ayoub, a spokesman for the Lebanese army, said that Sunday’s protesters were supporters of Ahmed Asaad, a Shiite politician who has taken a rare stand against Hezbollah. Head of the Lebanese Option party, Asaad ran unsuccessfully for parliament in 2009.

A photograph of the clashes taken by the Reuters news agency shows men in black shirts with yellow armbands — the color of Hezbollah’s flag — apparently attacking a man in a white shirt and jeans who appears to be unarmed. Another photo shows the men smashing the windows of a bus.

By midafternoon, streets around the embassy were quiet, and about 50 soldiers were stationed nearby. Witnesses were reluctant to speak.

One resident, who spoke on the condition of anonymity because he feared reprisals, said the embassy’s security guards had opened fire but only shot in the air. A worker in a nearby grocery store agreed.

The army said in a statement that it is “pursuing” the person who opened fire and called on citizens to “express their political views on events in Lebanon and Syria by peaceful and democratic means and not to be driven by groups wanting to use violence as a means to achieve their ends.”

Despite mounting anger at Hezbollah’s involvement in Syria, the opposition has failed to mobilize a significant groundswell of protest. The demonstration at the embassy came on the same day as an anti-Hezbollah rally in Beirut’s central Martyrs’ Square, but the protesters numbered in the dozens rather than the hundreds that organizers had hoped for. Empty chairs were cleared away as speakers took the stage.

“I am a bit disappointed,” said Elie Abdelnour, a 28-year-old member of the National Liberal Party. “There were rumors of a counter-demonstration, and I think some people are scared.” Others, he said, are resigned to the status quo. “Before, people used to give everything, but now, the youth tends to look away.”