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.
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."
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 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.
The videos and images, not just of the massive cloud of smoke erupting in Beirut but the damage and devastation it is caused kilometres away, have triggered a new wave of shock and anxiety in Lebanon, which is already teetering on the brink of a catastrophic economic collapse.
Just hours before the blast, anti-government protesters had been scuffling with the security forces outside the ministry of energy, demanding yet again accountability from the country's leaders.
There have been grave warnings of hunger in the streets or a reigniting of sectarian conflict if the economy worsens.
And the blast will remind many of the bomb that killed Rafik Hariri. The Lebanese will be hoping that this latest blast will remain a human tragedy - an accident - and not a premeditated act.
Lebanon is experiencing political turmoil, with street demonstrations against the government's handling of the worst economic crisis since the 1975-1990 civil war.
Many blame the ruling elite who have dominated politics for years and amassed their own wealth while failing to carry out the sweeping reforms necessary to solve the country's problems. People have to deal with daily power cuts, a lack of safe drinking water and limited public healthcare.
There has also been tension on the border with Israel, which said last week that it had thwarted an attempt by Hezbollah to infiltrate Israeli territory. But a senior Israeli official has told the BBC that "Israel has no connection" to the Beirut blast.
The blast happened close to the scene of the huge car bombing which killed ex-PM Hariri. Tuesday's blast also came days before the long-awaited verdict in the trial at a special court in the Netherlands of four men accused of orchestrating the attack.