A large blast in the Lebanese capital, Beirut, has killed at least 100 people and injured more than 4000 others, BBC reports quoting the health minister.
It is not yet clear what caused the explosion. Videos show smoke billowing from a fire before the blast, which is followed by a mushroom cloud.
Hospitals are said to be overwhelmed and many buildings have been destroyed.
Lebanon's internal security chief said the blast happened in an area housing highly explosive materials.
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 a possible 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.
UK Prime Minister Boris Johnson tweeted: "The pictures and videos from Beirut tonight are shocking. All of my thoughts and prayers are with those caught up in this terrible incident.
"The UK is ready to provide support in any way we can, including to those British nationals affected."
Hadi Nasrallah, an eyewitness, told BBC, “I saw the fire, but I didn't yet know there was going to be an explosion. We went inside. Suddenly I lost my hearing because apparently I was too close. I lost my hearing for a few seconds, I knew something was wrong.
And then suddenly the glass just shattered all over the car, the cars around us, the shops, the stores, the buildings. Just glass was going down from all over the building.
Literally all over Beirut, people were calling each other from different areas kilometres away and they were experiencing the same thing: broken glass, buildings shaking, a loud explosion.
Actually we were shocked because usually when it happens, just one area will experience those happenings after an explosion, but this time it was all of Beirut, even areas outside of Beirut.