The collapse of his government had looked inevitable after a string of resignations and a Cabinet meeting during which several Cabinet members made it clear they would also quit.
Diab's announcement came even as security forces in central Beirut clashed again with protesters demanding an end to an entrenched political system widely seen as inept, corrupt and dominated by sectarian interests and family dynasties.
"I announce the resignation of the government," he said at the end of a speech in which he strove to cast himself as an external victim of a corrupt and selfish political leadership.
According to the health ministry, at least 160 people were killed in Lebanon's worst peacetime disaster, 6,000 were wounded and about 20 remained missing after the disaster blamed on official negligence.
Six days after the enormous chemical blast wreaked destruction across swathes of the capital and was felt as far away as the island of Cyprus, residents and volunteers were still clearing the debris off the streets.
International rescue teams with sniffer dogs and specialised equipment remained at work at the disaster’s charred "ground zero”, where the search was now for bodies and not survivors.
The Lebanese want heads to roll over the tragedy and are asking how a massive stockpile of volatile ammonium nitrate, a compound used primarily as a fertiliser, was left unsecured at the port for years.
Diab’s resignation was met with cars honking in the streets and celebratory fire in the northern city of Tripoli but was unlikely to meet the long-term expectations of the Lebanese population.
The country’s top officials have promised a swift and thorough investigation — but they have stopped short of agreeing to an independent probe led by foreign experts as demanded by the protesters.
The outgoing government is now due to stay on in a caretaker capacity, and the next elections could still be months away.
Violent protests
Before Diab’s announcement, four ministers had already decided they could no longer serve a government that has shown little willingness to take the blame or to put state resources at the service of the victims.
At least nine lawmakers have also announced they would quit in protest, as have two senior members of the Beirut municipality.
The August 4 blast, which drew comparisons with the Hiroshima atom bomb, was so enormous that it altered the shape of not only Beirut’s skyline but even its Mediterranean coastline.
But it remained to be seen whether the disaster will also have a lasting impact on Lebanon’s entrenched post-war political culture, whose masters are widely seen as being bent on self-preservation and buck-passing.
During consecutive evenings of protests, the rage sparked by the explosion that disfigured Beirut and scarred so many of its residents had not relented.
"The resignation of ministers is not enough. Those who are responsible for the explosion should be held accountable,” said Michelle, a demonstrator in her early twenties, on Sunday.
She carried a poster of a friend who was killed in the blast, inscribed with the message "My government killed me”.
‘Direct’ aid
Demonstrators lamented that security forces were using tear gas against blast victims instead of helping them clean their wrecked homes and find a roof.
"We need an international investigation and trial to tell us who killed our friends and all the other victims, because they might try to conceal the truth,” said Michelle.
French President Emmanuel Macron supported the idea when he visited last Thursday, but his calls for reform and transparency appeared to receive less attention among Lebanese officialdom than his offer to raise aid money.
An online emergency support conference Macron chaired on Sunday, attended virtually by a slew of world leaders including US President Donald Trump, came up with pledges for more than 250 million euros.
Macron, who was given a hero’s welcome when he trudged through the rubble of ravaged old Beirut to meet distressed residents, stressed the aid would go "directly” to the population.
Many Lebanese are sceptically waiting to see how the aid delivery will navigate a sophisticated and deeply entrenched system of local and sectarian patronage organised by the country’s party barons.
Lebanese aid groups have warned foreign donors that any financial assistance risked being syphoned away.
The Beirut disaster compounded what has become Lebanon’s annus horribilis, deepening a dire economic crisis which had dragged half of the country into poverty in recent months.
The obliteration of the port and its huge grain silos in a country hugely reliant on imports has sparked fears of food shortages in the coming weeks.
Adding to Lebanon’s woes, coronavirus cases are reaching new highs almost every day, putting further strain on hospitals that are treating blast victims and the dozens wounded in the repression of the protests