Mikati told reports that "the estimated number is very high and may reach one million" -- which would amount to roughly a sixth of Lebanon's population.
"It is the largest displacement movement that may have happened... in Lebanon," he said.
On Friday, Israel killed Hezbollah's powerful leader Hassan Nasrallah in a move many fear risks destabilising Lebanon and the wider region.
Since Monday, intense Israeli attacks across Lebanon's east, south and on southern Beirut have killed hundreds of people and forced many to flee their homes.
Earlier this week, UN refugee chief Filippo Grandi said "well over 200,000 people are displaced inside Lebanon" and more than 50,000 had fled to neighbouring Syria.
The intensive strikes come as Israel shifted the focus of its operation from Gaza to Lebanon, after nearly a year of cross-border fire with Hezbollah over the Gaza war, with the group saying it is acting in support of ally Hamas.
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