Vice President Kamala Harris and second gentleman Doug Emhoff spoke with the parents of Hersh Goldberg-Polin, an American-Israeli hostage who was found dead in Gaza on Saturday along with five other Oct. 7 hostages, according to the Israel Defense Forces.

The vice president and her husband called parents Rachel Goldberg and Jon Polin on Sunday to "express our condolences following the brutal murder of their son by Hamas terrorists," Harris said in a statement on X.

"My heart breaks for their pain and anguish," Harris continued.

"I told them: As they mourn this terrible loss, they are not alone. Our nation mourns with them," Harris said.

Protesters took to the streets of Tel Aviv Sunday night, demanding a cease-fire agreement and the release of the remaining hostages being held by Hamas terrorists.

The demonstration came a day after Israel Defense Forces recovered the bodies of six hostages in tunnels under the city of Rafah in the Gaza Strip.

Protesters were seen waving Israeli flags as they demanded a cease-fire agreement, chanting "Deal. Now."

-ABC News' Victoria Beaule

At least six people were killed on Sunday when Israel Defense Forces conducted an airstrike on a school in Gaza City, according to Gaza Civil Defense.

The IDF said in a statement that the strike was aimed at Hamas terrorists they allege were operating a command-and-control center inside the Safad school to plan and carry out terrorist attacks against the IDF and Israel.

"Prior to the strike, numerous steps were taken to mitigate the risk of harming civilians, including the use of precise munitions, aerial surveillance, and additional intelligence," the IDF said in a statement.

The Gaza Civil Defense confirmed the Safad school was hit in the IDF strike, but said the school houses displaced people from the Al-Zeitoun area east of Gaza City.

-ABC News' Victoria Beaulé

Secretary of State Antony Blinken spoke Sunday with the parents of U.S.-Israeli citizen Hersh Goldberg-Polin, who Isreal Defense Forces found fatally shot Saturday along with five other hostages who were being held in Gaza tunnels by Hamas terrorists, according to a State Department spokesperson.

In his phone conversation Sunday with Rachel Goldberg and Jon Polin, Blinken offered "deep condolences for the death of their son," said State Department spokesperson Matt Miller said.

-ABC News' Shannon Kingston

Israel's national trade union, representing the majority of the country's unionized workers, announced it will hold a one-day general strike on Monday in an attempt to pressure the Israeli government to agree to a cease-fire and hostage deal.

Members of the Histadrut labor federation plan to walk off jobs across the nation at 6 a.m. local time Monday, said union chief Arnon Bar-David.

The union, comprised of 800,000 members, said the "Israeli economy will stop" during the strike.

Bar-David said the strike will prompt Israel's international airport to suspend operations at 8 a.m. Monday, including all takeoffs and landings.

"I will not tolerate abandonment [of hostages]," Bar-David said at a news conference on Sunday.

The Histadrut union is affiliated with dozens of sub-unions, including civil service, nurses, engineers, lawyers and industrial workers, officials said.

-ABC News' Dana Savir