A large blast in the Lebanese capital, Beirut, has killed at least 50 people and injured more than 2,700 others,the health minister says.
Videos show smoke billowing from a fire, then a mushroom cloud following the blast at the city's port.
Hospitals are said to be overwhelmed and many buildings have been destroyed.
Lebanon's internal security chief said the blast occurred in an area where highly explosive materials were stored. The exact cause is not yet clear.
A BBC journalist at the scene reported dead bodies and severe damage, enough to put the port of Beirut out of action.
The explosion comes at a sensitive time for Lebanon, with an economic crisis reigniting old divisions. Tensions are also high ahead of Friday's verdict in a trial over the killing of ex-Prime Minister Rafik Hariri in 2005.
Officials are pointing to an accident rather than a deliberate act as the likely cause.
The interior minister said first reports suggested what he called explosive material stored at the port had blown up.
Prime Minister Hassan Diab called it a catastrophe and said those responsible must be held to account.
He spoke of a "dangerous warehouse" which had been there since 2014, but said he would not pre-empt the investigation.
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.
"All the buildings around here have collapsed. I'm walking through glass and debris everywhere, in the dark," one witness near the port told AFP news agency.
The blast was heard 240km (150 miles) away on the island of Cyprus in the eastern Mediterranean.
President Michel Aoun called for an emergency meeting of the Supreme Defence Council, the presidency said on Twitter. Wednesday has been declared a day of mourning.