The blast happened at around 6pm local time (3pm UK), the BBC reports, near the city’s port.
Footage posted on social media shows a building ablaze, before a massive explosion sends a huge mushroom cloud into the sky.
The shockwave rocked much of the city, blowing out windows, flipping over cars, and causing several buildings near the site to collapse.
https://twitter.com/CNN/status/1290917959087665153
It is feared hundreds could yet be trapped beneath rubble.
President Michel Aoun said the blast was caused by thousands of unsafely stored ammonium nitrate, commonly used as a fertiliser – and makeshift explosive.
Prime minister Boris Johnson said the British government stood ready to support Lebanon "in any way it can".
Foreign secretary Dominic Raab said: "My thoughts and prayers are with those affected by the devastating explosion in Beirut today.
"The UK stands in solidarity with the people of Lebanon and is ready to offer help and support, including to those British nationals impacted."
The Foreign Office confirmed a number of its staff in Beirut sustained non-life threatening injuries in the explosion.
"The British Embassy Beirut is in contact with local authorities and urgently seeking further information following large explosions in the port area of Beirut on 4 August," said the FCO.
"There is significant disruption across Beirut. If you’re in Beirut, you should avoid the immediate area of the incident, remain vigilant, follow the advice of the local authorities and stay up to date with developments."
While travel to Beirut and the rest of Lebanon from the UK is currently suspended, the city and the rest of the country had – in recent years – become an increasingly popular tourist destination.
A number of operators have offered tours of the city and country either as standalone trips, or as part of a wider exploration of important sites in the Middle East.
