Gaza militants fired a salvo of more than two dozen rockets at southern Israel on Wednesday, prompting a sharp warning from Israeli Prime Minister Benjamin Netanyahu.
It was the heaviest barrage of rocket fire since November 2012.
The rocket fire was claimed by the armed wing of the radical Islamic Jihad movement a day after three of its militants were killed in an Israeli air strike on southern Gaza.
An Israeli security source told AFP militants had fired 25 rockets "in close succession all over the south" while the army put the number at "more than 20 rockets" saying that another three were intercepted by the Iron Dome anti-missile system.
Israel public radio put the number of rockets fired at 55.
Netanyahu linked the attacks to Tuesday's air strike targeting Islamic Jihad militants and pledged to act against those seeking to harm Israel "with great force", a statement from his office said.
"We will continue to strike those who want to harm us, we'll act against them very forcefully," he was quoted as saying by his spokesman Ofir Gendelman on Twitter.
Israeli police said the rocket fire fell in several waves along the length of Israel's border with Gaza and that one struck near a public library in the town of Sderot, while another hit near a petrol station in another area.
There was no immediate report of casualties.
"This is the biggest attack on Israel since the 2012 Operation Pillar of Defence," the military said on its Twitter feed, referring to a major eight-day confrontation in November 2012 which claimed the lives of 177 Palestinians, mostly civilians, and six Israelis.
In Gaza, Islamic Jihad's armed wing, the Al-Quds Brigades, said the salvo was in response to Israel's "aggression" in reference to Tuesday's air strike.