Opening an emergency cabinet meeting, President Aoun said: “No words can describe the horror that has hit Beirut, turning it into a disaster-stricken city”.
The explosion occurred just after 18:00 (15:00 GMT) after a fire at the port, and it was felt 240km (150 miles) away on the island of Cyprus, in the eastern Mediterranean.
Rescue workers in Lebanon are searching for dozens of people missing after a huge explosion devastated the port area of the capital, Beirut.
The blast, which shook the whole city, killed at least 135 people and injured more than 5000 others. A two-week state of emergency has been declared.
President Michel Aoun said the blast was caused by 2,750 tonnes of ammonium nitrate stored unsafely in a warehouse.
All port officials have been put under house arrest pending an investigation.
Customs chief Badri Daher told local media that his agency had repeatedly called for the ammonium nitrate to be removed, but “this did not happen, and we leave it to the experts to determine the reasons”.
Ammonium nitrate is used as a fertiliser in agriculture and as an explosive.
What triggered the explosion?
The ammonium nitrate had reportedly been in a warehouse in Beirut port for six years after it was unloaded from a ship impounded at the port in 2013.
The head of Beirut port and the head of the customs authority both told local media that they had written to the judiciary several times asking that the chemical be exported or sold on to ensure port safety.
The port’s General Manager Hassan Koraytem told that they had been aware that the material was dangerous when a court first ordered it stored in the warehouse, “but not to this degree”.