The second blast sent an enormous orange fireball into the sky, flattened the harbourside and drove a tornado-like shockwave through the city, shattering windows kilometres (miles) away.
Bloodied and dazed wounded stumbled among the debris, glass shards and burning buildings in central Beirut as the health ministry reported 73 dead and 3,700 injured and the minister bemoaned "a disaster in every sense of the word".
A soldier at the port, where relatives of the missing scrambled for news of their loved ones, told AFP: "It's a catastrophe inside. There are corpses on the ground. Ambulances are still lifting the dead."
The blasts were heard throughout the small country and as far away as Nicosia on the eastern Mediterranean island of Cyprus, 240 kilometres (150 miles) away.
"It was like an atomic bomb," said Makrouhie Yerganian, a retired schoolteacher in her mid-70s who has lived near the port for decades.