BEIRUT, Lebanon -- A protester was shot and killed on Sunday near the Iranian Embassy in Beirut, apparently in a clash between supporters and opponents of Hezbollah, the powerful Lebanese Shiite Muslim organization whose military intervention in the Syrian civil war has strained Lebanon's stability.
Demonstrators said that they were attacked by men wearing yellow armbands, the color of Hezbollah's flag. They said the men beat on the windows of a bus that was taking the demonstrators to the protest, and then opened fire.
Hezbollah's television station, Al-Manar, said that "a citizen" had shot a protester, citing statements form the Lebanese Army. The station did not say whether Hezbollah supporters were involved.
The victim was identified by fellow protesters as Hisham Salman, an organizer of a marginal Shiite faction with roots in the country's old feudal elite, which has tried to challenge Hezbollah's pre-eminence in Lebanese Shiite political life. He was quickly hailed as a martyr on Facebook pages that support the uprising against the government of the Syrian president, Bashar al-Assad; Hezbollah supports Mr. Assad.
The demonstrators in Beirut accused Hezbollah supporters of reacting violently to what was shaping up to be a relatively small protest of several dozen people near the embassy of Iran, Hezbollah's main foreign backer. Analysts said there was growing anxiety in Lebanon that Hezbollah's involvement in Syria could lead to a violent backlash against Shiites or create dissension in the wider Shiite community.
Hezbollah's core followers in Lebanon have been unwavering in their support for the group's recent escalation of its role in Syria, even as dozens of Hezbollah fighters have been killed or injured fighting in Syria against fellow Arab Muslims -- a new kind of battle for a group that was founded to fight Israel. Last week, Hezbollah fighters helped the Syrian government seize the strategic crossroads town of Qusayr, near the Lebanese border, from rebels who had held it for more than a year.
But the wider Lebanese population is deeply divided between supporters of Mr. Assad and backers of the uprising against him. As the Syrian conflict, which began as a struggle over political rights, drags on and becomes ever more sectarian in tone, it is awakening tensions in Lebanon, which still bears the scars of its own sectarian civil war. Many Sunni Muslims in Lebanon back the mostly Sunni Syrian rebels, while Hezbollah's followers increasingly feel threatened by the extremists among the rebels, who consider Shiites to be infidels.
Sunday's clash, though, was said to be between rival Shiite factions, a relatively rare occurrence given Hezbollah's status as the country's strongest political party and military force.
The organizer of the protest was Ahmad al-As'ad, the son of a former Parliament speaker and the founder of Lebanese Option, which vies with Hezbollah for Shiite votes but has never won a seat in Parliament. He said the demonstration's purpose was to send a message to Iran. "If you want to keep President Assad, it's fine," Mr. As'ad said he wanted to say to Tehran. "But why are you sending our young men to Syria? The Shiites are not there to be sacrificed."
He said that Hezbollah supporters had deliberately killed Mr. Salman, though other witnesses said the scene was chaotic. According to witnesses' accounts, several of the attackers drew guns and embassy guards fired in the air. A separate protest in downtown Beirut on Sunday drew several hundred more anti-Hezbollah demonstrators, mainly from secular and Sunni groups. The government sent armored vehicles to take up positions and keep order there. Hezbollah's main Sunni rivals did not bring out their supporters in large numbers; hundreds of chairs that had been set up in the square stood empty.
Mr. As'ad said his demonstrators were unarmed Shiites "who dared to come and protest" on behalf of "many others who are against Hezbollah policy but can't express it publicly."
Hezbollah is widely credited with having empowered Lebanon's long-downtrodden Shiite majority by providing jobs, education and a strong political voice; Mr. As'ad, by contrast, is descended from wealthy Shiite landowners and has struggled to attract broad support.
Hezbollah has portrayed the fight in Syria as necessary to preserve Hezbollah's ability to strike at Israel, by keeping open the conduit for arms from Iran that Mr. Assad's government provides, and to protect Lebanon from extremists among the Syrian rebels. In an indication of the threat, such extremists were accused by fellow opponents of Mr. Assad on Sunday of killing a 15-year-old boy in Aleppo, in northern Syria, after the boy invoked the name of the Prophet Muhammad in a way that the extremists considered blasphemous.
Rami Abdulrahman of the Syrian Observatory for Human Rights, an anti-Assad watchdog group based in Britain, gave an account of the boy's death that he said came from a member of his group who witnessed some of the events.
He said Islamist rebels overheard the boy, who came from a poor family and sold coffee from a street cart, as he argued with some friends who wanted him to lend them money. The boy told his friends he would not do so "even if Muhammad comes back to life, " Mr. Abdulrahman said, adding that "he just used a common Syrian expression that people use when they are adamant about something."
The gunmen beat and tortured the boy, he said, and then brought the boy to a public square, where they declared that those who insult the prophet deserve the same punishment as infidels. He said they spoke in classical Arabic, possibly indicating that they were foreigners. In front of a crowd that included the boy's parents, Mr. Abdulrahman said, the men shot the boy in the head and neck.
"His mother is veiled," he added. "She went up to them, uncovered her hair and begged them to leave him and kill her instead."
Mr. Abdulrahman provided what he said was a photograph of the boy, with what appeared to be a bullet exit wound through his mouth.
Hwaida Saad reported from Beirut, and Hala Droubi from Dubai, United Arab Emirates. Anne Barnard contributed reporting from Beirut.
Correction: June 10, 2013, Monday
This article has been revised to reflect the following correction: Because of an editing error, an earlier version of this article referred incorrectly to Ahmad al-As'ad of the group Lebanese Option. Mr. As'ad is the son of a former speaker of the Lebanese Parliament; he did not hold that post himself.
This article originally appeared in The New York Times.