Regardless, some applaud the return of Hariri, who resigned last October under pressure from ongoing anti-authority protests in Lebanon, believing that his contacts and alliances with the West and Gulf countries will help Lebanon escape its worst economic crisis since the country's 1975-1990 civil war.
Mr Hariri's dramatic and controversial return was secured by a slim majority - just 65 of out a total 120 votes - amid deep divisions over the shape of the cabinet he is expected to form.
"To the Lebanese in despair, I say: I am determined to deliver on my promise to end the collapse in our economy, society and security, to rebuild what the blast at the port destroyed and to form a government quickly", Mr Hariri said after accepting the job.
Hariri will hold the position for the fourth time despite months of widespread protests calling for a radical change to how the country is governed.
"We are in a disaster, how are we still allowing this to happen?"
There was no immediate reaction in the streets from the protest movement, which had however long said it did want the return of the political old guard it accuses of ineptitude and corruption. Her business was blown up in the August blast and she has since sent her daughter overseas fearing the country is not fit to live in.
"Now is a time for solidarity and we need a rescue government".
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"The more we wait, the more the boat sinks", he said. Hariri got backing from the other Shiite group, Amal, as well as a small Christian party and independents.
Hezbollah MP Mohammad Raad said that "national understanding" was needed to "preserve Lebanon" and that it would work to create a "positive atmosphere" to support the Cabinet formation process.
Any new government will have to contend with a financial meltdown worsening by the day, a COVID-19 outbreak and the fallout of the massive explosion at Beirut port that killed almost 200 people in August.
In the year since, Lebanon's currency sank, losing almost 80 percent of its value, while prices, unemployment and inflation soared.
Lebanese citizens have also been unable to access their savings after banks slapped on capital controls fearing a run on deposits. Tens of thousands of people were left homeless and the damage costs millions of dollars.
France, Lebanon's most supportive Western ally, has called for its leaders to form a more technocratic government to build stability that in turn would allow it to gain more global support. Diab also faced an economic crisis that pushed half the population into poverty due to the COVID-19 pandemic during his administration.
"We are also seeing a worrying search in cases coming through the airport from overseas", he added.