A car bomb rocked south Beirut, 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 international community was swift to condemn the attack Tuesday, with the UN Security Council renewing its appeal for all groups to stay out of the Syrian war despite growing cross border attacks.
The United States condemned the bombing "in the strongest possible terms," State Department spokeswoman Jen Psaki said.
"We strongly support the efforts of the Lebanese armed forces and the internal security forces to restore stability and maintain calm in Beirut and throughout Lebanon and we condemn any activity that targets or puts at risk civilians," Psaki added.
A witness told AFP the Beirut 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."
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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.
The European Union also condemned the bombing and said it showed the need for unity.
"This appalling act of violence underlines the need for all Lebanese to maintain their national unity and actively work to preserve peace, safety and stability in Lebanon," said Michael Mann, spokesman for the EU's foreign policy chief Catherine Ashton.
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.