Residents have reported homes being completely destroyed, as well as windows being blown out and ceilings dropping.
Lebanon analyst David Daoud said that he thought it was unlikely that Israel was involved in the explosion, saying that Israel has no interest in escalation at this point.
Authorities say it was caused by highly explosive materials at the port, but didn't say whether it was an accident or an attack.
The governor of Beirut port told Sky News that a team of firefighters at the scene had "disappeared" after the explosion.
A major explosion in Lebanon's capital of Beirut shattered windows and rocked buildings for hundreds of feet, according to reports. There were hundreds of casualties, with bodies buried under the rubble, officials said.
The health minister of Lebanon said a ship carrying fireworks blew up in the city port.
Several witnesses present in Beirut said that this was the largest and most devastating explosion they had felt.
Four alleged members of the Shiite Muslim fundamentalist group Hezbollah are on trial in absentia at the court in the Netherlands over the huge Beirut suicide bombing that killed Sunni billionaire Hariri and 21 other people.
The blast came at a time when Lebanon's economy is facing collapse, hit both by a financial crisis and coronavirus restrictions.
A picture shows the scene of an explosion in Beirut on August 4, 2020.
A woman in the city centre Tuesday told AFP the blast "felt like an earthquake" and "bigger than the explosion in the assassination of Rafic Hariri in 2005".