Officials are blaming highly explosive materials stored in a warehouse for six years.
President Michel Aoun tweeted it was “unacceptable” that 2,750 tonnes of ammonium nitrate was stored unsafely.
An investigation is under way to find the exact trigger for the explosion. Lebanon’s Supreme Defence Council said those responsible would face the “maximum punishment” possible.
Hospitals are said to be overwhelmed and many buildings have been destroyed.
President Aoun declared a three-day mourning period, and said the government would release 100 billion lira (£50.5m; $66m) of emergency funds.
A BBC journalist at the scene reported dead bodies and severe damage, enough to put the port of Beirut out of action.
Rescue workers still struggling to treat thousands of people wounded in an enormous explosion that rocked Beirut turned their attention on Wednesday morning to the desperate search for survivors.
The blast, so powerful it could be felt more than 150 miles away in Cyprus, leveled whole sections of the city near the port of Beirut on Tuesday evening, leaving nothing but twisted metal and debris for blocks in Beirut’s downtown business district. It capsized a docked passenger ship, shattered windows miles away and registered on seismographs, shaking on the earth as strongly as a 3.3-magnitude earthquake.
The waterfront neighborhood, normally full of restaurants and nightclubs, was essentially flattened. A number of crowded residential neighborhoods in the city’s eastern and predominantly Christian half were also ravaged.
Nearly all the windows along one popular commercial strip had been blown out and the street was littered with glass, rubble and cars that had slammed into each other after the blast. The buildings that remained standing looked as if they had been skinned, leaving hulking skeletons
Maintenance work likely led to the explosion of a large cache of ammonium nitrate that devastated a large part of the Lebanese capital last week, a U.S. government assessment concluded, according to a person familiar with the matter.
The assessment adds to the emerging picture of what caused the Aug. 4 blast that killed more than 160 people, with at least 60 others still missing. The explosion has stoked public outrage toward Lebanon’s government, with many protesters demanding justice after the explosive material was stored for years in the port adjacent to the heart of Beirut, and calling for their country’s leaders to be held accountable for years of poor governance and corruption.
After days of protests that forced Prime Minister Hassan Diab’s cabinet to resign earlier this week, Lebanon’s parliament in its first session since the explosion approved Thursday a state of emergency that grants sweeping powers to the military.
The state of emergency allows the armed forces to try civilians in military courts for security crimes, ban public gatherings, censor media, set curfews and enter residents’ homes for security purposes. A state of emergency had been declared by the cabinet immediately after the blast but the parliamentary approval means it can stay in place until Aug. 21.