A live video interview conducted by a BBC journalist captured the horrifying moment yesterday's massive explosion hit Beirut, showing the reporter reacting to the initial explosion before being hit by the devastating blast wave.

Maryam Toumi, from BBC News Arabic, was conducting a video interview when the explosion at Beirut's port shook the city.

A rumbling sound is heard and Toumi reacts by nervously standing up.

Just seconds later a deafening roar is heard and Toumi and the camera are thrown to the floor by the shockwave.

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As the camera keeps rolling, alarms go off and Toumi can be heard screaming.

Her interview subjects look on in shock at the unfolding horror.

The video is one of many from Beirut that show the moment of the explosion, showing daily life interrupted by the apocalyptic blast.

In one video, posted to Twitter, the shockwave can be seen racing across the water in a split-second before slamming into a car, shattering the windows and badly damaging the vehicle.

A video shot from a boat offshore shows the mushroom cloud towering above the port in the moment of explosion before a shockwave hits the boat before the video cuts out.

Investigators began searching through the wreckage of Beirut's port for clues to the cause of the massive explosion that ripped across the Lebanese capital, and the government ordered port officials put under house arrest amid speculation that negligence was to blame.

The investigation is focusing on how 2750 tons of ammonium nitrate, a highly explosive chemical used in fertilisers, came to be stored at the facility for six years, and why nothing was done about it.

A massive explosion rocked downtown Beirut on Tuesday, flattening much of the port, damaging buildings and blowing out windows and doors as a giant mushroom cloud rose above the capital. Video / Twitter

The explosion at the port killed at least 135 people and wounded about 5000, Health Minister Hamad Hassan said. Hospitals were overwhelmed — one that was damaged in the blast had to evacuate all its patients to a nearby field for treatment.

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Buildings were damaged for miles around the city, and Beirut's governor said that hundreds of thousands might not be able to return to their homes for two or three months.

It was the worst, most destructive single explosion to strike Lebanon in a history filled with destruction, in a 1975-1990 civil war, conflicts with Israel and periodic terror attacks.

- Additional reporting, AP