The disaster was preceded by a large fire at the Port of Beirut, on the city's northern Mediterranean coast. In videos posted on social media white smoke could be seen billowing from Warehouse 12, next to the port's huge grain silos.

Shortly after 18:00 (15:00 GMT), the roof of the warehouse caught alight and there was a large initial explosion, followed by a series of smaller blasts that some witnesses said sounded like fireworks going off.

About 30 seconds later, there was a colossal explosion that sent a mushroom cloud into the air and a supersonic blastwave radiating through the city.

That blastwave leveled buildings near the port and caused extensive damage over much of the rest of the capital, which is home to two million people. Hospitals were quickly overwhelmed.

"What we are witnessing is a huge catastrophe," said the head of the Lebanese Red Cross, George Kettani. "There are victims and casualties everywhere."

Beirut Governor Marwan Abboud said as many as 300,000 people had been made temporarily homeless and that collective losses might reach $10-15bn (£8-11bn).

The explosion's shockwave blew out windows at Beirut International Airport's passenger terminal, about 9km (5 miles) away from the port.

The blast was also heard as far away as Cyprus, about 200km (125 miles) across the Mediterranean Sea, and seismologists at the United States Geological Survey said it was the equivalent of a 3.3-magnitude earthquake.

Lebanon's President, Michel Aoun, blamed the detonation on 2,750 tonnes of ammonium nitrate that he said had been stored unsafely at a warehouse in the port.

Ammonium nitrate is a crystal-like white solid commonly used as a source of nitrogen for agricultural fertiliser. But it can also be combined with fuel oils to create an explosive used in the mining and construction industries. Militants have made bombs with it in the past.

Experts say that ammonium nitrate is relatively safe when stored properly. However, if you have a large amount of material lying around for a long time it begins to decay.

"The real problem is that over time it will absorb little bits of moisture and it eventually turns into an enormous rock," Andrea Sella, professor of chemistry at University College London, told the BBC. This makes it more dangerous because if a fire reaches it, the chemical reaction will be much more intense.

Ammonium nitrate has been associated with deadly industrial accidents. In 1947, a ship carrying 2,000 tonnes of the chemical exploded in Texas, killing 581 people.