Lebanon was holding a day of mourning Friday after a car bomb killed at least 22 people in a Beirut stronghold of Shiite group Hezbollah, which backs Syria's embattled president.
A previously unknown group, apparently a Syrian rebel cell, said it carried out Thursday's attack in the densely populated southern suburbs of Beirut, between Bir al-Abed and Rweiss, districts where Hezbollah security is normally tight.
Police said the death toll had climbed to at least 22, while the Red Cross said 325 people were wounded.
The National News Agency reported that seven people, including a man and his three children, were missing.
On Friday morning, investigators were combing the scene for clues, as Hezbollah security personnel sealed off the area, an AFP photographer reported. Group members in civilian clothes stopped and searched cars in the southern suburbs.
The explosion, reminiscent of the frequent attacks during Lebanon's 1975-1990 civil war, sent a plume of black smoke into the Mediterranean sky, caused heavy damage to buildings and set several cars ablaze.
Leaders from across the political spectrum condemned the bombing, and a day of mourning was declared.
President Michel Sleiman said the "terrorist" bombing targeted all Lebanese, not just Hezbollah.
Former prime minister Saad Hariri, a Sunni leader and staunch critic of Hezbollah, said the attack was "part of a vicious terrorist scheme" targeting Lebanon.
UN chief Ban Ki-moon urged Lebanon's fractious political scene to stay united, in a statement that condemned the bombing as "completely unacceptable".
"During this period of heightened tensions, the secretary general urges all Lebanese to remain united, to rally around their state institutions and to focus on safeguarding Lebanon's security and stability," the statement said.
And the UN Security Council "strongly condemned the terrorist attack."
It called for "all Lebanese people to preserve national unity in the face of attempts to undermine the country's stability" and urged all parties "to refrain from any involvement in the Syrian crisis."
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The blast came a day after Hezbollah chief Hassan Nasrallah said his militant group was taking steps to ensure the security of the southern suburbs, after a July 9 car bomb in Bir al-Abed wounded dozens.
A witness told a Lebanese television channel he saw a van drive past three times before its driver found a parking spot where he set off the bomb.
The explosion had the impact of an "earthquake," another witness said.
Hezbollah is a key supporter of President Bashar al-Assad and has sent fighters across the border this year to bolster government forces, which have been battling a deadly anti-regime revolt since March 2011.
The movement has become a hated foe of Syria's rebels, most of whom are Sunnis, while Assad is a member of the Alawite offshoot of Shiite Islam.
An online video surfaced shortly after the attack showing three masked men, two of them holding rifles, in front of a white flag inscribed with the Islamic profession of faith.
"You, the pig Hassan Nasrallah, we send you our second powerful message because you haven't understood yet," said one member of the group calling itself the Battalion of Aisha, an apparent Sunni affiliation.
Thursday's blast comes six weeks after a car bomb attack in the same area wounded more than 50 people.
Another little-known Syrian rebel group, the Special Forces 313 Brigade, claimed that attack and said it was in revenge for Hezbollah fighting alongside the Assad regime.
The mainstream rebel Free Syrian Army condemned the latest bombing, as it did the July blast.
The FSA military command "condemns this criminal act targeting civilians," Louai Moqdad, the group's political coordinator, told AFP, adding that the three masked men did not claim to be Syrian rebels.
"He who takes part in the murder of the Syrian people alongside the criminal is responsible for this crime," he said.
Lebanon is deeply divided into supporters and opponents of the regime in neighbouring Syria. Sectarian tensions in Lebanon have soared as the Syrian conflict has raged on.
Nasrallah is scheduled to speak on Friday in an event marking the anniversary of a devastating summer 2006 war that pitted his fighters against the group's archfoe Israel.