A Lebanese firefighter extinguishes a fire at the site of an explosion in the Bir al-Abed area in the southern suburbs of Beirut on July 9. An explosion wounded at least 18 people, medical and security officials said. (WAEL HAMZEH/EPA)

A bomb detonated in the parking lot of a supermarket in one of Beirut’s mostly Shiite southern suburbs Tuesday, injuring dozens of people and stirring fears that sectarian tensions unleashed by the conflict in neighboring Syria are reaching Lebanon’s capital.

The Bir al-Abed area, where the explosion occurred, is a stronghold of the Shiite Hezbollah group, Lebanon’s most powerful political and military force. The attack followed weeks of rising tensions spurred by the group’s deepening entanglement in the Syrian war, including the dispatch of fighters to aid President Bashar al-Assad.

Live footage from the scene showed blazing fires, a huge cloud of smoke and a six-foot-wide crater carved into the ground. Health Minister Ali Hassan Khalil told the LBC1 television station Tuesday that 53 people had been injured and that 42 of them had later been released from hospital.

Al-Manar television station, which is owned by Hezbollah, reported that 18 people were injured, none seriously.

The parking lot where the bomb exploded is next to a supermarket called the Islamic Cooperation Center that would have been busy with shoppers buying supplies ahead of the upcoming month of Ramadan, when Muslims fast in the daytime and feast at night. Although the target of the blast was unclear, concerns have been widespread for months that Hezbollah’s role in the war would lead to retaliatory attacks against Hezbollah strongholds, and several Syrian rebel groups had issued such threats.

The leadership of the mainstream Free Syrian Army condemned the bombing as a “terrorist” attack and denied that it was involved. But a little-known Free Syrian Army battalion named Brigade 313-Special Missions claimed responsibility for the blast, saying in a posting on its Facebook page that the attack had targeted Hezbollah security officials as they headed to their cars after attending a meeting nearby.

The group also claimed it had carried out two earlier, smaller bombings in other parts of the country that had appeared to target Hezbollah officials traveling to Syria and vowed that this attack “will not be the last.”

“We will hit everywhere in Lebanon and all the strongholds of the Lebanese Hezbollah . . . until it ends its collaboration in shedding Syrian blood,” the statement said.

A spokesman for the brigade who uses the pseudonym Osama al-Homsi said he was unable to offer proof of the group’s claim. But he said the Brigade, which has units fighting in several parts of Syria, was made up of defected army officers with expertise in manufacturing such bombs.

The claims could not be verified, but if true, they herald a potentially new complication for the Obama administration as it explores ways of arming the rebels through the Supreme Military Council headed by Gen. Salim Idriss. Homsi said Brigade 313 recognizes the leadership of the Free Syrian Army but does not follow its orders or receive supplies from it. Louay al-Moqdad, the political and media representative of the Free Syrian Army, said he did not believe the brigade had carried out the attack.

U.S. Ambassador to Lebanon Maura Connelly issued a statement “strongly condemning” the bombing and calling on Lebanon’s factions to “exercise calm, constraint and respect for Lebanon’s security and stability.”

Underlining the danger that the tensions could escalate, some people in the mostly Sunni northern Lebanese town of Tripoli unleashed volleys of celebratory gunfire when they heard the news, according to a Lebanese army officer who spoke on the condition of anonymity because he is not authorized to talk to reporters.

Sporadic fighting has already erupted between Sunnis and Shiites or between Lebanese army forces and Sunni radicals in Tripoli, in the southern town of Sidon and in eastern Lebanon.

But Beirut, the capital, has largely remained immune from the tensions. Interior Minister Marwan Charbel visited the scene of the bombing there Tuesday and told reporters he believed the purpose of the attack was to “provoke sectarian . . . clashes between Sunnis and Shiites in Lebanon.”

“Neither the Sunnis nor the Shiites will fall for this,” he added.

Hezbollah lawmaker Ali Ammar called the attack “a despicable act of terrorism” and implied that he held Israel and the United States responsible, without providing evidence. Bir al-Abed is home to many top Hezbollah officials and offices and was almost completely destroyed by Israeli warplanes during the 2006 conflict. It has since been rebuilt.

Suzan Haidamous in Beirut contributed to this report.