The explosion happened at a petrol station in Hermel, a stronghold of the Shiite movement Hezbollah, Lebanon's interior minister said.
The Al-Nusra Front in Lebanon, a group named after Al-Qaeda's Syrian affiliate, claimed the attack on Twitter, saying it was a suicide bombing in response to Hezbollah's involvement in Syria.
It was the seventh attack to target Hezbollah in Lebanon since mid-2013, when the group sent men to Syria to fight alongside President Bashar al-Assad's troops against mainly Sunni rebel groups.
The United Nations Security Council issued a statement condemning the "terrorist attack" and calling for the perpetrators to be brought to justice.
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Council members also appealed to all Lebanese people "to preserve national unity in the face of attempts to undermine the country's stability" and for all parties to refrain from any involvement in the Syrian crisis.
In a separate statement the UN secretary general's spokesman Martin Nesirky said Ban Ki-moon also strongly condemned the car bombing and extended his condolences to the bereaved.
The explosion, which came at around 1600 GMT after nightfall, sparked a huge blaze that hindered the arrival of emergency services.
Al-Manar broadcast footage of a massive fire at the site.
Security forces later closed off the area and firefighters managed to extinguish the blaze.
The petrol station is part of a charitable network set up by Mohammed Hussein Fadlallah, a leading Shiite cleric and Hezbollah spiritual guide who died in 2010.