At least 27 people have been killed and more than 2,500 injured in a large blast in the Lebanese capital, Beirut, the health minister says.

It is not yet clear what caused the explosion in the port region. Videos posted online showed a column of smoke followed by a large mushroom cloud.

Hospitals are said to be overwhelmed and many buildings have been destroyed.

According to Lebanon’s internal security chief, the blast happened in an area housing highly explosive materials.

The explosion comes at a sensitive time for Lebanon, with an economic crisis reigniting old divisions. Tensions are also high ahead of August 7 verdict in a trial over the killing of ex-Prime Minister Rafik Hariri in 2005.

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Some reports suggest the explosion may have been an accident. Lebanon’s National News Agency reported a fire breaking out at what it called an explosives depot at the port before the explosion.

Local media showed people trapped beneath rubble. A witness described the first explosion as deafening, and video footage showed wrecked cars and blast-damaged buildings.

President Michel Aoun called for an emergency meeting of the Supreme Defense Council, the presidency said on Twitter.

August 5 has been declared a day of mourning, the Lebanese prime minister said.