BEIRUT -- A large suspected car bomb exploded Thursday in the southern part of this city in a neighborhood controlled by the militant group Hezbollah, signaling another encroachment of the Syrian civil war into Lebanon.
The blast killed as many as 20 people and wounded scores of others in streets crammed with offices and military posts for the Shiite Muslim group. Several 10-story apartment buildings burned for more than an hour.
Thursday's explosion was notable because it struck the busy residential Rweiss neighborhood. That is well within what Hezbollah considers its "security zone," an area where it maintains an overt and aggressive buffer around the homes and offices of its leadership.
A much-smaller car bomb wounded dozens of people last month in an attack that, like Thursday's, was widely seen as a spillover from the Syrian civil war and Hezbollah support for the regime of Syrian leader Bashar Assad.
A group that calls itself Brigades of Aisha, Mother of the Faithful -- a name with powerful sectarian overtones, reflecting a militant Sunni Muslim stance -- took credit for Thursday's explosion in a video released to local television news. "We've sent a message to [Hezbollah chief] Hassan Nasrallah's pigs," one masked gunmen said in a portion of the statement aired.
Three masked men could be seen posing with guns behind a white banner inscribed with the Muslim profession of faith. The group threatened more attacks.
Celebratory gunfire was reported at nightfall in some Sunni neighborhoods across the nation, highlighting the deep sectarian divide in a country where many Sunnis support the Syrian rebellion and Shiites typically back the Assad government.
Last month, U.S. intelligence officials warned the Lebanese government that Sunni militants, ideologically aligned with al-Qaida, planned to target Hezbollah for backing the Syrian regime. Factions in the Syria fighting have broken along increasingly sectarian lines, and those divisions have been bleeding across the border into Lebanon for much of the last year.
Fearful of becoming a target for radical Sunni groups -- who consider Hezbollah and other Shiites to be heretics -- Hezbollah recently added armed patrols and random traffic stops around the neighborhood where the explosion hit Thursday.
The blast came at evening rush hour from what Lebanese security officials and Hezbollah security men at the scene suspected was a car bomb. It tore through a dense block of apartments, including one building rumored to hold a Hezbollah facility.
The bomb detonated in a packed shopping street, killing 18 people and injuring 291, according to the state news agency, which cited caretaker Public Health Minister Ali Hasan Khalil. The Reuters news agency put the number of dead at 20.
"We were working the dinner hours when there was a huge explosion that blew out our back windows," said a young man who works at a fast-food restaurant. He refused to give his name because of the number of plainclothes Hezbollah security men visible in the area. "I ran out into the street and saw many dead bodies, and everything was on fire."
A large cloud of oily black smoke was visible from miles away. An hour after the explosion, fire crews appeared to be struggling to contain a series of blazes within apartment buildings next to the blast site. Many people flocked to the scene to claim bodies. Some worried that more dead or wounded might be trapped in the rubble.
Men who appeared to be Hezbollah security agents could be seen arresting a young man for questioning. "He's Syrian. That's enough," said one of those arresting the man. "Any Syrian found in the security zone is subject to arrest tonight."
The rumor that the apartment block contained a Hezbollah facility -- such outposts are often low-key but ubiquitous in the area -- gained credence by the Hezbollah security force's reaction to the blast. Its men immediately cordoned off the area and occasionally fired guns in the air to warn curious onlookers to back away.
Security in the area had been high, suggesting that a suicide attack might be a plausible explanation for how a bomb could penetrate the neighborhood.
The Washington Post contributed.