Pictures show great devastation in the city, with blown out windows and landslides. At least 78 people have been killed and more than 4,000 injured, according to Lebanese Health Minister Hamad Hasan. And the death toll is expected to rise.
The Lebanese Red Cross says hundreds of injured people have been taken to city hospitals, but many are still trapped in their destroyed homes.
The Prime Minister has triggered a national day of mourning today, reports Reuters.
The President: "Unacceptable"
Lebanon's internal security chief Abbas Ibrahim tells Reuters that the explosions took place in the city's port area, in a part where highly explosive material has been stored for a number of years. But exactly what was the igniting spark has not emerged.
On Twitter, President Michel Aoun writes that it is unacceptable that 2,750 tonnes of ammonium nitrate have been stored for six years without proper security, reports the news agency Reuters.
Michel Aoun has called the country's Security Council to a crisis meeting and Prime Minister Hassan Diab says those responsible will be held accountable.
- This will not go unnoticed. Those responsible will pay the price, he says.Trump: "Horrible attack"
But US President Donald Trump says the explosions appear to have been "some kind of bomb", despite the fact that the Lebanese authorities have not clarified what was the cause of the explosion. He says it is information he has received from the US military.
- We will be available for help. It looks like a terrible attack, he says.
"My house is completely destroyed"
Hotel dieu hospital states that it has received 500 injured and can no longer receive more, reports the news company LBCI. Many have to be operated on and the hospital says that there is a great need for blood.
According to the TV channel Al Arabiya, the sound must have been heard up to 20 kilometers north of the city.
- I was standing on the balcony, it felt like an airplane hit my house. Doors and windows are completely blown away, my house is completely destroyed. The city is full of sirens and covered in red smoke, there is chaos in the streets, said Beirut resident Muhammad.