A huge explosion in a port warehouse district near the center of Beirut killed more than 40 people, injured over 2,500 others and sent shockwaves across the Lebanese capital on Tuesday, shattering windows and causing apartment balconies to collapse.

Officials expected the death toll to rise sharply as emergency workers dug through rubble across a swathe of the city to rescue people and remove the dead. It was the most powerful blast to hit Beirut in years, making the ground tremble.

“What we are witnessing is a huge catastrophe,” the head of Lebanon’s Red Cross George Kettani told broadcaster Mayadeen. “There are victims and casualties everywhere – in all the streets and areas near and far from the explosion.”

Three hours after the blast, which struck shortly after 6 p.m. (1500 GMT), a fire still blazed in the port district, casting an orange glow across the night sky as helicopters hovered and ambulance sirens sounded across the capital.

A security source said victims were being taken for treatment outside the city because Beirut hospitals were already packed with wounded. Red Cross ambulances from the north and south of the country and the Bekaa valley to the east were called in to cope with the huge casualty toll. 

The blast was so big that some residents in the city, where memories of heavy shelling during the 1975 to 1990 civil war live on, thought an earthquake had struck. Dazed, weeping and, wounded, people walked through streets searching for relatives.

Lebanon’s interior minister said initial information indicated highly explosive material, seized years ago, that had been stored at the port had blown up. The minister later told Al Jadeed TV ammonium nitrate had been in storage there since 2014.

Footage of the explosion shared by residents on social media showed a column of smoke rising from the port district followed by an enormous blast, sending a ball of white smoke and fireball into the sky. Those filming the incident from high buildings 2 km (more than a mile) from the port were thrown backwards by the shock.

Lebanon’s health minister said more than 25 people had been killed and more than 2,500 were injured. Lebanon’s Red Cross said hundreds of people had been taken to hospitals.