The blast struck Beirut's port and surrounding area with the force of a 3.5 magnitude earthquake.
The authorities say a warehouse storing Amonium Nitrate caught fire - its a substance which can be used as a fertilizer and as an explosive.
The blast comes at a sensitive time for Lebanon the economy is in collapse and sectarian tensions are on the rise again - what triggered the fire is a matter of speculation.
Shocked and bloodied residents staggered into the streets, thousands of people have been injured and scores are dead. The Prime minister has declared a day of mourning.
Hassan Diab, Lebanese Prime Minister says those responsible for this catastrophe will pay the price.
Hospitals are said to be overwhelmed as they already have to cope with the COVID-19 pandemic.
International help and condolences have been offered and an investigation launched into why such a dangerous substance was stored close to the city.