The death of a Palestinian hunger striker in Israeli custody triggered a barrage of rockets fired from Gaza into residential areas.
"This is an initial response to this heinous crime," said a coalition of Gaza-based Palestinian militant groups, led by the enclave's militant Hamas rulers.
They were responding to news that Khader Adnan, 45, had been found dead in his cell almost three months into a hunger strike.
Palestinian Prime Minister Mohammed Shtayyeh called it an "assassination," accusing Israel of medical neglect.
Air raid sirens sounded and Israeli municipal councils opened public bomb shelters. Field fires broke out. Shrapnel punched holes into pavement, shattered windows and charred cars. Four rockets were intercepted by Israel's Iron Dome missile defence system, authorities said.
The Israeli military responded with tank fire into Gaza and said it was planning further retaliation. A military official, speaking on condition of anonymity under regulations, said that the army assessed that Islamic Jihad fired the rockets with Hamas' knowledge.
"The security establishment will act with determination and force against anyone who tries to harm the citizens of Israel," said Defense Minister Yoav Gallant.
Hundreds gathered at Adnan's home in the West Bank town of Arraba, near Jenin, holding posters bearing his image and calling for revenge. In a rare call for de-escalation, Adnan's widow, Randa Musa, asked Palestinians not to respond with violence.
"We do not want a single drop of blood shed," she told the crowd. "We do not want rockets to be fired."
Khader Adnan, 45, a leader of the militant Islamic Jihad group, helped introduce the practice of protracted hunger strikes by individual prisoners as a form of protest. Palestinian detainees have mostly used hunger strikes to challenge administrative detention, a controversial tactic in which more than 1,000 Palestinians and a handful of Israelis are currently being held without charge or trial.
Adnan first grabbed international headlines and inspired solidarity protests over a decade ago, when he staged a 66-day hunger strike against his administrative detention. That galvanised hundreds of other prisoners to join the strike, which ended with a deal for his release. He was later arrested again.