A car bomb rocked south Beirut on Tuesday, injuring at least 53 people in the most serious incident in the stronghold of Lebanon's Shiite Hezbollah movement since the start of the Syrian conflict.
The blast comes amid spiralling tension in Lebanon over the civil war raging in Syria, where Hezbollah fighters have joined President Bashar al-Assad's forces in facing down a revolt by mainly Sunni rebels.
"A car bomb exploded near a commercial cooperative called the Islamic Cooperation Centre in Bir al-Abed," which lies in the heart of Hezbollah's Beirut stronghold, the military source said.
Lebanon's Health Minister Ali Hassan Khalil told AFP that 53 people were wounded, adding that 12 remained in hospital and two had undergone surgery.
The densely populated Bir al-Abed neighbourhood is home mainly to Shiite Muslims.
Several broadcasters, among them Hezbollah's Al-Manar, showed firefighters battling several blazes while large clouds of black smoke billowed into the sky.
A witness told AFP the explosion was "huge".
"Everyone started panicking. Everyone was running left and right" after the blast, said Carole Mansour, who owns a shoe shop near the affected area.
"The smoke was so (thick)," Mansour told AFP, adding that Hezbollah members dressed in civilian clothing were quick to deploy around the site of the blast.
"I started following the sounds of the screams of people. My employees ran to the site to try to see what was happening because they have relatives there," she added.
"I can't believe someone would do this on the first day of Ramadan," said Mansour, referring to the Muslim holy fasting month.
Some Shiites started their fasting on Tuesday, although other Shiites and Sunnis will begin fasting either Wednesday or Thursday.
Lebanese politicians from across the spectrum quickly condemned the blast, including President Michel Sleiman who called for an end "to such tactics... and respect for the security of all Lebanese citizens."
Former prime minister and opposition leader Saad Hariri, much of whose Sunni constituency in Lebanon backs the Syrian uprising and has been angered by Hezbollah's intervention, warned that the country must "avoid sliding into wars that will only mean more division for Lebanon."
Officially neutral in Syria's conflict, Lebanon is deeply divided into pro- and anti-Assad camps.
Hezbollah and its allies back Assad, who adheres to the Alawite offshoot of Shiite Islam, while the Sunni-led opposition supports rebels seeking his ouster.
In an indication of the extent of the political divisions, Interior Minister Marwan Charbel was attacked by Hezbollah supporters as he arrived at the scene, an AFP photographer said.
Hezbollah officials then fired live rounds into the air to disperse the protesters, who were apparently opposed to the visit by Charbel, a member of Lebanon's anti-Assad camp.
Tuesday's blast is not the first time Hezbollah's Beirut stronghold has come under attack in apparent retaliation for its role in the Syrian conflict.
In late May, two rockets fired from inside Lebanon landed in southern Beirut, wounding four people just hours after Hezbollah's chief Hassan Nasrallah pledged to fight until victory for Syria's regime.
Syrian rebels have threatened to target Hezbollah over its involvement in the conflict, but the Syrian opposition National Coalition condemned Tuesday's blast.
"Targeting civilians is a criminal act that is opposed to the ideas and the principles of the revolution," the group said in a statement.
In Sunni-majority Tripoli meanwhile, armed men celebrated the Beirut blast, firing live rounds into the air, a security source said.
Tripoli has seen frequent, deadly clashes pitting pro-uprising Lebanese Sunnis against pro-regime Lebanese Alawites.
Elsewhere, five children were injured when a bomblet exploded in Wadi Khaled, which lies in northern Lebanon on the Syrian border, a security source said.
The source said the children were playing in their school playground when the device exploded, adding that the bomblet had likely been found by one of the children and taken to the school grounds.