Jim Muir in Beirut: "This is the biggest and by far the most deadly attack so far"
At least 16 people have been killed by a large blast in a southern suburb of the Lebanese capital, Beirut, with some 200 more reported wounded.
The area contains strongholds of the Shia militant movement Hezbollah.
The explosion happened in a densely populated part of the capital. Heavy damage was reported to nearby buildings and cars.
The blast is being linked to the conflict in Syria, which has worsened sectarian tensions in Lebanon.
Plumes of smoke rose over the area where the blast occurred, between the Bir Abed and Rweiss districts of the city.
Lebanese officials said that the blast was the result of a car bomb.
The site of the blast is close to the Sayyed al-Shuhada complex, where Hezbollah often holds mass rallies, although there does not seem to have been an obvious target.
The BBC's Jim Muir, at the scene of the blast, says the bomb appeared to have gone off between two residential buildings and he saw chaotic scenes, with fire engines and ambulances piling into a narrow road.
Smoke was still rising from nearby buildings more than two hours after the blast, our correspondent reports.
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This is a densely populated area of Beirut
One injured man at the scene told Reuters news agency: "I don't know what happened. It's as if we were struck by an earthquake."
Lebanon's outgoing prime minister has declared Friday a day of mourning for the victims.
Few would doubt the blast was a consequence of Hezbollah's involvement in the conflict in Syria, our correspondent says.
Fighters from the militant group were instrumental in a strategic victory by Syrian government forces in Qusair, close to the border with Lebanon, in early June.
Hezbollah politicians denounced the latest bomb as a "terrorist attack" and called for restraint from their followers.
A previously unknown group called the Battalions of Ayesha said it carried out the attack. In a video message posted online, they also implied they had been behind another bomb blast in a nearby area last month which injured dozens.
The group threatened to carry out more attacks, referring to Hezbollah strongholds as "colonies of Iran".
Events in Syria are putting Lebanon's fragile peace in jeopardy,correspondents say, threatening the equilibrium which has held since the end of the civil war more than 20 years ago.
