Hezbollah has said two of its fighters and an eight-year-old girl have been killed. The Lebanese health ministry says eight people have been killed and at least 2,750 injured.
Iran's ambassador to Lebanon - Mojtaba Amani - is reportedly among the injured. Iranian State TV reports his wounds are "superficial" and that he is "conscious and in no danger".
Hezbollah's chief, Hassan Nasrallah, was not harmed in the blasts, according to Reuters which quotes a senior Hezbollah source.
Unverified CCTV footage circulating on social media appears to show the blasts, including one video of what appears to be an explosion in a man's trouser pocket as he stood at a shop till.