President Michel Aoun said the blast was caused by 2750 tonnes of ammonium nitrate stored unsafely in a warehouse.
Many have accused the authorities of corruption, neglect and mismanagement.
The blast killed at least 137 people and injured about 5000 others, while dozens are still missing. A two-week state of emergency has begun.
French architect Jean-Marc Bonfils, involved in rebuilding the city after the civil war, and firefighter Sahar Fares, one of the first responders at the scene, were among the first fatalities to be named. A German diplomat was also among the dead.
French President Emmanuel Macron - the first world leader to visit since the tragedy - was mobbed as he walked through the city today, with residents imploring him to help and denouncing their leaders.
"Help us, you are our only hope," one resident called out. "Please don't give money to our corrupt government," said another, before adding: "We can't take this any more."
At a press conference, Macron said a new political order was needed in Lebanon. "The anger I saw in Beirut today also showed signs of hope for the future," he said.
An aid conference for Lebanon would be announced in the coming days, he said. France would organise aid along with the European Union and the World Bank and would make sure it was sent directly to relief organisations working on the ground.
Funding was available, he said, but political reforms had to take place before it could be sent.
He vowed that there would be no blank cheques for Lebanon's leaders, but cautioned that he could not tell the Lebanese government what to do. France is the former colonial power in Lebanon.
He also called for an international investigation into the explosion "to prevent things from remaining hidden and doubt from creeping in", he said.
An audit of Lebanon's central bank was also needed - "If there is no audit of the central bank, in a few months there will be no more imports and then there will be a lack of fuel and of food," Macron said.
Filmmaker Jude Chehab told the BBC: "Beirut is crying, Beirut is screaming, people are hysterical and people are tired."
Chadia Elmeouchi Noun, a resident currently in hospital, said: "I've known all the time that we are led by incompetent people, incompetent government... But I tell you something - what they have done now is absolutely criminal."
Yesterday, the government announced that a number of port officials were placed under house arrest pending an investigation into the explosion.
The country's Supreme Defence Council insisted that those found responsible would face the "maximum punishment".
Meanwhile, Amnesty International and Human Rights Watch (HRW) have called for an independent investigation into the blast. In a statement, HRW said it had "serious concerns about the ability of the Lebanese judiciary to conduct a credible and transparent investigation on its own".