“The Sunni-Shiite conflict is in the open now, it’s been triggered and operationalized,” said Paul Salem, director of the Carnegie Middle East Center in Beirut. “This is a deep wound, and it’s going to have serious repercussions if it’s not immediately and seriously addressed.”

Image Hussein al-Haj Obaid, a Sunni Muslim, was abducted and tortured by gunmen loyal to Hezbollah, the Shiite militant group. The blood is boiling here, he said. Credit... Bryan Denton for The New York Times

Lebanese political leaders have tried hard to avoid stirring sectarian sentiment, emphasizing the religious diversity of both the governing coalition and the Hezbollah-led opposition movement. In a speech delivered the day before Hezbollah supporters seized the capital, the group’s leader, Hassan Nasrallah, went out of his way to deny that Sunni-Shiite tensions were an issue.

But after Hezbollah supporters humiliated Lebanon’s main Sunni political leader, Saad Hariri  crushing his weak militia, forcing his party’s television station off the air and burning two of his movement’s buildings  many of Mr. Hariri’s supporters were enraged, and they said they would look to another Sunni leader who would help them fight back.

That sentiment has stirred fears that moderate, secular Sunni leaders like Mr. Hariri could lose ground to more radical figures, including the jihadists who thrive in Lebanon’s teeming Palestinian refugee camps. Fatah al Islam, the radical group that fought a bloody three-month battle with the Lebanese Army in a refugee camp in northern Lebanon last year, issued a statement Thursday condemning Hezbollah’s actions. The group also gave a warning: “He who pushes our faces in the dirt must be confronted, even if that means sacrificing our lives and shedding blood.”

A New Kind of Conflict

The Sunni-Shiite conflict is relatively new in Lebanon, where the long civil war that ended in 1990 revolved mostly around tensions between Christians and Muslims, and their differences over the Palestinian presence in the country. But after Iran helped establish Hezbollah in the early 1980s, Lebanon’s long-marginalized Shiites steadily gained power and stature. They have also grown in numbers. Although there has been no census since 1932, Shiites are widely believed to be more numerous than Sunnis or Christians, the country’s other major groups.

Tensions began to rise in 2005 after Syrian troops ended their long occupation of Lebanon, leaving the country’s factions to broker a power-sharing agreement. Hezbollah established a crucial alliance with Michel Aoun, a former general and one of the country’s most powerful Christian leaders, to oppose the Western-backed government of Prime Minister Fouad Siniora, a Sunni.

In late 2006, sectarian street battles began taking place in mixed Sunni-Shiite neighborhoods, mostly among young followers of Mr. Hariri’s Future Movement and the Amal Party, a Hezbollah ally. The fighting was prompted by hard feelings after Hezbollah’s withdrawal from the cabinet and its subsequent campaign to bring down Mr. Siniora, who refused to step down despite the resignation of all the cabinet’s Shiite ministers.