Palestinian Arabs, in an October surprise that no one will welcome, early Saturday morning at the Gaza Strip launched rockets toward Israel, setting off air raid sirens and putting the country on a sudden war footing. 

Palestinian militants at Gaza also carried out an unprecedented infiltration into southern Israel, sending dozens of terrorists across the border and firing thousands of rockets into the country as the ruling Hamas terrorist group announced the beginning of a new operation.

Israel’s defense minister said Hamas has started a war against Israel and pledged that “Israel will win.”

Following a security cabinet meeting at the Israeli military headquarters at Tel Aviv, the defense minister, Yoav Gallant, warned that Hamas “made a grave mistake” in launching barrages of rockets into southern and central Israel in its surprise morning attack.

“The state of Israel will win this war,” Mr. Gallant said.

By mid-morning Saturday at least five people are thought to have been killed and at least 100 hundred injured, the Times of Israel reported. There are reports of multiple casualties and damages to a small number of buildings at Ashkelon and Tel Aviv, both cities along Israel’s Mediterranean coast.

In response, Israel’s military put the country on war alert and began striking targets at Gaza, setting the stage for what was likely to be a new heavy round of fighting between the bitter enemies. In a serious escalation, salvos of rockets fired from Gaza set off constant air raid sirens as far north as Jerusalem.

The sound of outgoing rockets whooshing through the air could be heard at Gaza, and sirens could be heard as far away as Tel Aviv during the early morning barrage. Videos posted on social media showed what appeared to be uniformed Palestinian gunmen inside the Israeli border town of Sderot.

A video from Gaza showed what looked like the lifeless body of an Israeli soldier being trampled by an angry crowd of Palestinians shouting, “God is Great.” Other footage appeared to show terrorists dragging away an Israeli soldier, still alive, on a motorcycle and Palestinian men dancing atop an Israeli tank that had been set ablaze. The authenticity of the videos could not immediately be verified.

The elusive leader of Hamas’s military wing, Mohammed Deif, announced the beginning of what he called “Operation Al-Aqsa Storm.”

“Today the people are regaining their revolution,” he said in the recorded message, as he called on Palestinians from Jerusalem to northern Israel to join the fight and “expel the occupiers and demolish the walls.”

“We must set the earth on fire under the feet of the occupiers,” Mr. Deif said, claiming that Hamas had fired more than 5,000 rockets into Israel. There was no way immediately to verify that claim.

The escalation comes at a delicate time for Prime Minister Netanyahu’s government, as protests against the government’s plan to overhaul the judiciary have spread even within the ranks of the military.

Yet protests against the judicial overhaul scheduled for Saturday night were abruptly canceled. “We stand with the residents of Israel and give full support to the IDF and the security forces,” the organizers of the Brothers and Sisters in Arms protest group said in a statement. “We call on all those who are needed to report [for duty] and play their part to safeguard the security and health of  the residents of Israel.”

The Hamas terrorist operation, coming on the major Jewish holiday of Simchat Torah, caught Israel off-guard, raising memories of the 1973 war in which Israel’s enemies launched a surprise attack on Yom Kippur.

The Israeli military confirmed Saturday that an infiltration had occurred at several locations near the Gaza border in southern Israel. It ordered residents to remain indoors.

“In the last hour, the Hamas terrorist organization had begun a massive shooting of rockets from the Gaza Strip into Israeli territory, and terrorists infiltrated into Israeli territory in a number of different locations,” it said.

Cities and towns in southern Israel emptied as the military closed roads near Gaza and millions of Israelis scrambled to reach bomb shelters while air raid sirens wailed overhead. In the kibbutz of Nahal Oz, just 2.5 miles from the Gaza Strip, terrified residents who were huddled indoors said they could hear constant gunfire echoing off the buildings.

The army said residents next to Gaza should stay in their homes due to the infiltration.

“With rockets we somehow feel safer, knowing that we have the Iron Dome [missile defense system] and our safe rooms. But knowing that terrorists are walking around communities is a different kind of fear,” a 42-year-old volunteer firefighter at Nahal Oz, Mirjam Reijnen, said, adding that she and her three kids have been too scared to leave the shelter even for a moment to use the bathroom.