The blast killed at least 100 people and injured more than 4,000 others.
The whole city was shaken by the explosion and a mushroom cloud could be seen spreading over the port area.
President Michel Aoun said the blast was caused by 2,750 tonnes of ammonium nitrate stored unsafely in a warehouse.
Ammonium nitrate is used as a fertiliser in agriculture and as an explosive.
President Aoun declared three days of mourning which started on Wednesday. Opening an emergency cabinet meeting, he said: "No words can describe the horror that has hit Beirut last night, turning it into a disaster-stricken city".
"Amid last night's smoke, flames and destruction, I would like to laud the zeal of the Lebanese who rushed to the blast location and perimeter and the hospitals to offer support and assistance," he added.
The explosion occurred just after 18:00 (15:00 GMT) on Tuesday after a fire at the port.
Eyewitness Hadi Nasrallah says that he saw the fire but did not expect the blast. "I lost my hearing for a few seconds, I knew something was wrong, and then suddenly the glass just shattered all over the car, the cars around us, the shops, the stores, the buildings. Just glass going down from all over the building," he told the BBC.
BBC Arabic reporter Maryem Taoumi was conducting a video interview in Beirut at the time of the explosion.
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