President Michel Aoun said 2,750 tons of ammonium nitrate, used in fertilizers and bombs, had been stored for six years at the port without safety measures.
Some local TV stations reported the blast was inside an area where firecrackers were stored.
Soon after the explosions took place a photo emerged purportedly showing workers welding a warehouse door next to half-tonne bags of ammonium nitrate, with netizens suggesting it was this type of behaviour that could have led to the disaster.
As Beirut's 4 million residents wake to the full horror and scale of the damage to their city, lives, and livelihoods, questions will be asked about why such large quantities of the unsafe chemical were allowed to be stored in the middle of the city without adequate safety measures, and who is responsible.
The huge explosion in port warehouses near the city centre as killed more than 73 people, injured over 2,750 and sent shockwaves that shattered windows, smashed masonry and shook the ground.
People walked around in shock, with helicopters overhead and teams searching for the missing at sea.
It was not immediately clear what caused the initial blaze on Tuesday that set off the blast.
Footage of the explosion posted on social media showed a column of smoke rising from the port, followed by an enormous blast, sending a white mushroom cloud and fireball into the sky. Those filming the incident from tower blocks as far as 2 km (one mile) from the port were thrown backwards by the shockwave.
The prime minister called for a day of mourning.
The explosion came three days before a UN-backed court is due to deliver a verdict in the trial of four suspects from the Shi'ite Muslim group Hezbollah over a 2005 bombing that killed former Prime Minister Rafik al-Hariri and 21 others.
Western countries including the United States, Britain and France also said they were ready to assist.
The port district was left a tangled wreck, disabling the nation's main route for imports needed to feed a nation of more than 6 million people.
Residents said glass was broken in neighbourhoods on Beirut's Mediterranean coast and inland suburbs several kms (miles) away.