Two huge explosions rocked the Lebanese capital Beirut on Tuesday, killing and wounding dozens of people, shaking buildings and sending huge plumes of smoke billowing into the sky.

Video footage of the second blast showed an enormous orange fireball that dwarfed nearby buildings and sent a devastating tornado-like shockwave ripping through the city.

"We heard an explosion, then we saw the mushroom," said one resident who witnessed the second, deafening explosion from her balcony in the city's Mansourieh district.

"The force of the blast threw us backwards into the apartment," she said.

Lebanese media carried images of people trapped under rubble, many bloodied, after the massive blasts, the cause of which was not immediately known.

A soldier at the port, who asked not to be named, told AFP: "It's a catastrophe inside. There are corpses on the ground. Ambulances are still lifting the dead."

The official National News Agency confirmed deaths in the blast, without citing a number.

The explosions "caused dozens of injuries," a security source said.

The AFP correspondent said every shop in the Hamra commercial district had sustained damage, with entire storefronts destroyed, windows shattered and many cars wrecked.

Injured people were walking in the street, while outside the Clemenceau Medical Centre, dozens of wounded people, many covered in blood, were rushing to be admitted to the centre, including children.

Destroyed cars had been abandoned in the street with their airbags inflated.

A huge cloud of black smoke was engulfing the entire port area, the AFP correspondent said.

The port zone was cordoned off by the security forces, allowing access only to a string of ambulances, fire trucks and people whose relatives were working inside the devastated area.

A huge blaze was burning at the port, where ambulances were rushing away the wounded, their sirens wailing.

The blasts were heard as far away as Nicosia on the eastern Mediterranean island of Cyprus, 240 kilometres (150 miles) away.

- 'Like an earthquake' -

"Buildings are shaking," tweeted one resident, while another wrote: "An enormous, deafening explosion just engulfed Beirut. Heard it from miles away."

Online footage from a Lebanese newspaper office showed blown out windows, scattered furniture and demolished interior panelling.

The explosions came at a time when Lebanon is suffering its worst economic crisis in decades, which has left nearly half of the population in poverty.

Lebanon's economy has collapsed in recent months, with the local currency plummeting against the dollar, businesses closing en masse and poverty soaring at the same alarming rate as unemployment.

The explosions also come as Lebanon awaits the verdict on Friday on the 2005 murder of former Lebanese premier Rafic Hariri, killed in a huge truck bomb attack.

Four alleged members of the Shiite Muslim movement Hezbollah are on trial in absentia at a court in the Netherlands over the huge Beirut bombing that killed Sunni billionaire Hariri and 21 other people.

A woman in the city centre Tuesday told AFP: "It felt like an earthquake ... I felt it was bigger than the explosion in the assassination of Rafic Hariri in 2005".

Tensions have also been high with neighbouring Israel, after Israel said it thwarted an infiltration attempt by up to five Hezbollah gunmen, a claim denied by the Lebanon-based and Iran-backed group.