The Israeli military said more than 100 projectiles were fired early Sunday from Lebanon, forcing hundreds of thousands to take cover and prompting school closures in Israel's north.
The military said that "approximately 20 projectiles were identified crossing from Lebanon" shortly before 5:00 am (0200 GMT), followed by a barrage of "approximately 85 projectiles" launched from Lebanon after 6:00 am (0300 GMT).
"Hundreds of thousands of people had to take refuge in bomb shelters at that time across northern Israel," military spokesman Lieutenant Colonel Nadav Shoshani told AFP.
The military in an earlier statement said the rocket fire sparked fires, while Israel's medical emergency service said at least four people suffered "shrapnel injuries," three of whom in the area of the northern Israeli city of Haifa.
Israel's civil defense agency has ordered all schools in Israel's north closed following the rocket fire, the latest escalation in nearly a year of cross-border exchanges throughout the Gaza war.
Educational activities would not be permitted across northern Israel until at least Monday at 6:00 pm (1500 GMT), the military's Home Front Command said, affecting "hundreds of thousands of children" according to Shoshani.
"In Haifa, a lot of schools are closed... and offices are empty," said resident Patrice Wolff, who works in the medical industry.
He told AFP there was "more and more pressure" coming from Hezbollah as well as from Israeli forces on the Lebanese group.
The military said it launched strikes on Hezbollah targets in southern Lebanon in response to the rocket fire.
Shoshani said the military had hit a range of targets over the past day, mostly "rockets launchers and rocket launcher barrels."
The Israeli strikes were meant "to prevent a larger-scale attack," the military spokesman told an online press briefing.
A steady escalation in hostilities between Israel and Hezbollah has stoked fears of all-out war.
Israeli officials this week have signaled their intention to turn the focus of military operations from the Hamas-ruled Gaza Strip to the Hezbollah in Lebanon.