Tel Aviv, Israel - A nationwide strike aimed at ramping up pressure on Israel's government to secure the release of the remaining hostages in Gaza was set to begin Monday.

The call by Israel's largest union to paralyze the economy follows a night of massive demonstrations, with tens of thousands of protesters taking to the streets in an outpouring of grief and fury over six hostages killed in the Gaza Strip.

Israeli soldiers recovered the bodies of the six "from an underground tunnel in the Rafah area" of southern Gaza on Saturday, the military said.

Relatives and demonstrators accused the government of not doing enough to bring them back alive, and called for an immediate ceasefire to rescue the dozens still captive.

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"We must stop the abandonment of the hostages... I have come to the conclusion that only our intervention can shake those who need to be shaken," said Histadrut union chair Arnon Bar-David.

"Starting tomorrow (Monday) at six in the morning, the entire Israeli economy will go on complete strike."

Of the 251 hostages seized during Hamas' October 7 attack, 97 remain captive in Gaza, including 33 the army says are dead.

Scores were released during a one-week truce in November, with campaigners and family members believing another deal is the best option to ensure the rest return.

"We are asking our government to stop everything and to make a deal," Yair Keshet, uncle of hostage Yarden Bibas, said during Sunday night's protest in Tel Aviv.

Critics have accused Prime Minister Benjamin Netanyahu of prolonging the war for political gain.

"Were it not for the delays, sabotage, and excuses" in months of mediation efforts, the six hostages "would likely still be alive," campaign group the Hostages and Missing Families Forum said in a statement.

During protests that convulsed Tel Aviv Sunday night, demonstrators marched past six symbolic coffins draped with the Israeli flag and carried pictures of the deceased hostages.

A handful of protesters clashed with police while some burned tires on a blocked highway where they defied water cannons.

The six hostages were identified as Carmel Gat, Eden Yerushalmi, Almog Sarusi, Ori Danino, US-Israeli Hersh Goldberg-Polin, and Russian-Israeli Alexander Lobanov.

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Military spokesperson Daniel Hagari said all six "were abducted alive on the morning of October 7" and "brutally murdered by Hamas terrorists shortly before we reached them."

Qatar-based Hamas official Izzat al-Rishq said they were "killed by Zionist (Israeli) bombing," an accusation the military denied.

Israeli health ministry spokesperson Shira Solomon said the hostages were "murdered by Hamas terrorists with several close-range gunshots," about 48-72 hours before their autopsies.

A senior Hamas official told AFP on condition of anonymity that "some" of the six had been "approved" for release in a potential hostage-prisoner swap under a deal yet to be agreed.

"This is not how imagined it would end, Eden, my love," Yerushalmi's mother tearfully told mourners at a funeral in the central city of Petah Tikva. "I wanted so bad to have you back alive."

US President Joe Biden said he was "devastated and outraged" by the hostages' deaths, but "still optimistic" about sealing a ceasefire deal in mediation efforts along with Qatar and Egypt.

In the besieged Gaza Strip, rescuers were digging through rubble for people buried alive by Israeli strikes on a school sheltering displaced people in Gaza City.

Civil defense agency spokesperson Mahmud Bassal told AFP that Sunday's strike on the Safad School killed 11 people.

Israel's military claimed it had hit a Hamas command center.

Nearly 11 months of siege have flattened much of Gaza and destroyed its water, sanitation, and medical facilities, contributing to the spread of preventable diseases.

Across Gaza, a series of "humanitarian pauses" are expected to facilitate a vaccination drive after the first confirmed polio case in 25 years.

The vaccination campaign formally launched on Sunday at three health centers in central Gaza, according to Yasser Shaaban, director of Al-Awda hospital.

Philippe Lazzarini, head of the UN's Palestinian refugee agency, called it a "race against time to reach just over 600,000 children."

"For this to work, parties to the conflict must respect the temporary area pauses," he posted on social media.

Israel has killed at least 40,738 people in Gaza since October, according to the territory's health ministry. The UN rights office says most of the dead are women and children.

In the occupied West Bank, at least 24 Palestinians have been killed since Israel launched raids on Wednesday, including 14 who militant groups claimed as members.

On Saturday, the UN Office of the High Commissioner for Human Rights called on Israel to immediately end its attack on the Jenin refugee camp, saying it had received "information that Israeli soldiers are rounding up dozens of young Palestinians from their homes and interrogating them, as well as subjecting them to various forms of mistreatment, including beatings."

An AFP photographer saw Israeli bulldozers in the Jenin city center a day after an official said soldiers had destroyed streets and power and water had been cut off in the adjacent camp.

"(We live in) terror and fear for the children," said Jenin resident Adel Marai Egbaria. "No one dares to go out."

Further south near the Tarqumiya checkpoint, Israeli police said a "shooting attack" killed three officers on Sunday.

According to the UN, at least 637 Palestinians have been killed in the West Bank by Israeli troops or settlers since the Gaza war began.

Twenty-three Israelis, including soldiers, have been killed in Palestinian attacks or during army operations over the same period, according to official figures.

In July, the International Court of Justice ruled Israel's decades-long occupation of the Palestinian Territories, including East Jerusalem, to be illegal.