AT least 100 people are dead and more than 4000 injured in a massive explosion which rocked central Beirut, the head of Lebanon's Red Cross has said.

The blast damaged buildings and blew out windows and doors as a giant mushroom cloud rose above the capital city.

A civil defence official on the scene of the blast said his men had evacuated dozens to hospitals and that there were still bodies inside the port, many of them under debris.

Lebanon's PM Hassan Diab tweeted that the explosion was caused by more than 2700 tonnes of ammonium nitrate left unsecured at a warehouse near the city's port for six years.

It was stunning even for a city that has been shaken by civil war, suicide bombings and bombardment by Israel.

Dozens of ambulances ferried the injured from the port area, where the wounded lay on the ground. Meanwhile, hospitals called for blood donations.

Video taken by residents showed a fire raging at the port, sending up a giant column of smoke, illuminated by flashes of what appear to be fireworks. Local TV stations reported that a fireworks warehouse was involved.

The fire then appeared to catch at a nearby building, triggering a more massive explosion, sending up a mushroom cloud and a shock wave over the city.

“It was like a nuclear explosion,” said Walid Abdo, a 43-year-old school teacher in the neighbourhood of Gemayzeh near Beirut.

Charbel Haj, who works at the port, said it started as small explosions like firecrackers, then the huge blast erupted and he was thrown off his feet. His clothes were torn.

Miles from the port, balconies were knocked down, windows shattered, streets were covered with glass and bricks and lined with wrecked cars. Motorcyclists picked their way through traffic, carrying the injured.

The blast came at a time when Lebanon’s economy is facing collapse, hit both by a financial crisis and coronavirus restrictions. Many have lost jobs, while the worth of their savings has evaporated as the currency has plunged in value against the dollar. The result has thrown many into poverty.

It also comes amid rising tensions between Israel and the militant Hezbollah group along Lebanon’s southern border.

The explosion was reminiscent of massive blasts during Lebanon’s civil war and took place only three days before a UN-backed tribunal was set to give its verdict in the killing of former prime minister Rafic Hariri in a truck bombing more than 15 years ago.

That explosion, with a ton of explosives, was felt miles away, just like Tuesday’s explosion.

An Israeli government official later said his country “had nothing to do” with the blast.

Israel and Lebanon’s Hezbollah militant group are bitter enemies, and tensions have been high following a series of recent clashes.