BEIRUT - A huge blast in port warehouses near central Beirut storing highly explosive material killed 78 people, injured nearly 4,000 and sent shockwaves that shattered windows, smashed masonry and shook the ground across the Lebanese capital.

Officials said they expected the death toll to rise further after Tuesday's blast as emergency workers dug through rubble to rescue people and remove the dead. It was the most powerful explosion in years to hit Beirut, which is already reeling from an economic crisis and a surge in coronavirus infections.

President Michel Aoun said that 2,750 tonnes of ammonium nitrate had been stored for six years at the port without safety measures and said it was "unacceptable".

He called for an emergency cabinet meeting on Wednesday and said a two-week state of emergency should be declared.

"What we are witnessing is a huge catastrophe," the head of Lebanon's Red Cross George Kettani told broadcaster Mayadeen. "There are victims and casualties everywhere."

Hours after the blast, which struck shortly after 6pm (11pm Singapore time), a fire still blazed in the port district, casting an orange glow across the night sky as helicopters hovered and ambulance sirens sounded across the capital.

A security source said victims were taken for treatment outside the city because Beirut hospitals were packed with wounded. Ambulances from the north and south of the country and the Bekaa valley to the east were called in to help.

The blast was so big that some residents in the city, where memories of heavy shelling during the 1975 to 1990 civil war live on, thought an earthquake had struck. Dazed, weeping and wounded people walked through streets searching for relatives.

Others sought their missing loved ones in the overflowing hospitals. One medic said 200 to 300 people had been admitted to a single emergency department. "I've never seen this. It was horrible," the medic, who gave her name as Rouba, told Reuters.

Prime Minister Hassan Diab told the nation there would be accountability for the deadly blast at the "dangerous warehouse".

"Those responsible will pay the price," he said in a televised address to the nation, adding that details would be made public.

The US embassy in Beirut warned residents in the city about reports of toxic gases released by the blast, urging people to stay indoors and wear masks if available.

Smoke and fireball

Footage of the explosion shared by residents on social media showed a column of smoke rising from the port followed by an enormous blast, sending up a white cloud and a fireball into the sky. Those filming the incident from high buildings 2km from the port were thrown backwards by the shock.

Officials did not say what caused the initial blaze that set off the blast. A security source and local media said it was started by welding work carried out on a hole in the warehouse.

The government said it as still struggling to establish the full scale of the disaster. "There are many people missing. People are asking the emergency department about their loved ones and it is difficult to search at night because there is no electricity," Health Minister Hamad Hasan told Reuters.

He said 78 people were killed and nearly 4,000 injured. "We are facing a real catastrophe and need time to assess the extent of damages."

The prime minister called for a day of mourning on Wednesday, and the country's banking association said banks would be closed.

Tangled wreckage

The explosion occurred 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 which killed former Prime Minister Rafik al-Hariri and 21 others.

Hariri was killed by a huge truck bomb on the same waterfront, about 2km from the port.

Israeli officials said Israel, which has fought several wars with Lebanon, had nothing to do with Tuesday's blast and said their country was ready to give humanitarian and medical assistance. Shi'ite Iran, the main backer of Hezbollah, also offered support, as did Tehran's regional rival Saudi Arabia, a leading Sunni power.

Western countries including the United States, Britain and France said they were ready to assist.

Images showed port buildings reduced to tangled masonry, devastating the main entry point to a country that relies on food imports to feed its population of more than 6 million.

It threatens a new humanitarian crisis in a nation that hosts hundreds of thousands of Syrian refugees and which is already grappling with economic meltdown under one of the world's biggest debt burdens.

Residents said glass was broken in neighbourhoods on Beirut's Mediterranean coast and inland suburbs several kilometres away. In Cyprus, a Mediterranean island 180km across the sea from Beirut, residents heard the blast. One resident in Nicosia said his house and window shutters shook.