The ruling Hamas militant group in the Gaza Strip carried out an unprecedented attack on Israel at daybreak Saturday, firing thousands of rockets as dozens of fighters infiltrated the heavily fortified border in several locations by air, land and sea, catching the country off guard on a major holiday.

Several hours after the invasion began, Hamas militants were still fighting gunbattles inside several Israeli communities in a surprising show of strength that shook the country. Israel’s national rescue service said at least 40 people have been killed and hundreds wounded, making it the deadliest attack in Israel in years.

At least 561 wounded people were being treated in Israeli hospitals, including at least 77 who were in critical condition, according to an Associated Press count based on public statements and calls to hospitals.

There was no official comment on casualties in Gaza, but AP reporters witnessed the funerals of 15 people who were killed and saw another eight bodies arrive at a local hospital. It was not immediately clear if they were fighters or civilians.

Social media was replete with videos of Hamas fighters parading what appeared to be stolen Israeli military vehicles through the streets and at least one dead Israeli soldier within Gaza being dragged and trampled by an angry crowd of Palestinians shouting “God is Greatest."

Videos released by Hamas appeared to show at least three Israelis captured alive. The military declined to give details about casualties or kidnappings as it continued to battle the infiltrators.

“We are at war,” Israeli Prime Minister Benjamin Netanyahu said in a televised address, declaring a mass army mobilization. “Not an ‘operation,’ not a ‘round,’ but at war.”

“The enemy will pay an unprecedented price,” he added, promising that Israel would “return fire of a magnitude that the enemy has not known.”

At a meeting of top security officials later on Saturday, Netanyahu said the first priority was to “cleanse the area” of enemy infiltrators, then to “exact a huge price from the enemy," and to fortify other areas so that no other militant groups join the war.

The serious invasion on Simchat Torah, a normally joyous day when Jews complete the annual cycle of reading the Torah scroll, revived painful memories of the 1973 Mideast war practically 50 years to the day, in which Israel’s enemies launched a surprise attack on Yom Kippur, the holiest day of the Jewish calendar.

Comparisons to one of the most traumatic moments in Israeli history sharpened criticism of Netanyahu and his far-right allies, who had campaigned on more aggressive action against threats from Gaza. Political commentators lambasted the government over its failure to anticipate what appeared to be a Hamas attack unseen in its level of planning and coordination.

The Israeli military struck targets in Gaza in response for some 2,500 rockets that sent air raid sirens wailing constantly as far north as Tel Aviv and Jerusalem, some 80 kilometers (50 miles) away. It said its forces were engaged in gunfights with Hamas militants who had infiltrated Israel in at least seven locations. The fighters had sneaked across the separation fence and even invaded Israel through the air with paragliders, the army said.

Israeli TV broadcast footage of explosions tearing through the Gaza-Israel border fence, followed by what appeared to be Palestinian gunmen riding into Israel on motorcycles. Gunmen also reportedly entered on pickup trucks.

It was not immediately clear what prompted Hamas to launch the attacks, which would have likely required months of planning.

But over the past year Israel's far-right government has ramped up settlement construction in the occupied West Bank, Israeli settler violence has displaced hundreds of Palestinians there, and tensions have flared around a flashpoint Jerusalem holy site.

The shadowy leader of Hamas’ military wing, Mohammed Deif, announced the start of what he called “Operation Al-Aqsa Storm." The Al-Aqsa mosque compound in Jerusalem is the third holiest site in Islam, and is located on the holiest site for Jews, who refer to it as the Temple Mount.

“Enough is enough,” Deif, who does not appear in public, said in the recorded message, as he called on Palestinians from east Jerusalem to northern Israel to join the fight. “Today the people are regaining their revolution."

In a televised address, Israeli Defense Minister Yoav Gallant warned that Hamas had made “a grave mistake” and promised that “the state of Israel will win this war."

Western nations condemned the incursion and reiterated their support for Israel, while others called for restraint on both sides.

“The U.S. unequivocally condemns the unprovoked attacks by Hamas terrorists against Israeli civilians,” said Adrienne Watson, spokeswoman for the U.S. National Security Council. “We stand firmly with the government and people of Israel and extend our condolences for the Israeli lives lost in these attacks.”

Watson said Jake Sullivan, the national security adviser, has spoken with his Israeli counterpart, Tzachi Hanegbi.

Saudi Arabia, which has been in talks with the U.S. about normalizing relations with Israel, released a statement calling on both sides to exercise restraint. The kingdom said it had repeatedly warned about “ the dangers of the situation exploding as a result of the continued occupation (and) the Palestinian people being deprived of their legitimate rights.”

The attack comes at a time of historic division within Israel over Netanyahu's proposal to overhaul the judiciary. Mass protests over the plan have sent hundreds thousands of Israeli demonstrators into the streets and prompted hundreds of military reservists to avoid volunteer duty — turmoil that has raised fears over the military’s battlefield readiness and raised concerns about its deterrence over its enemies.

The infiltration of fighters into southern Israel marked a major escalation by Hamas that forced millions of Israelis to hunker down in safe rooms. Cities and towns emptied as the military closed roads near Gaza. Israel's rescue service and the Palestinian Health Ministry in Gaza appealed to the public to donate blood.

“We understand that this is something big,” Lt. Col. Richard Hecht, an Israeli army spokesman, told reporters. He said the Israeli military had called up the army reserves.

Hecht declined to comment on how Hamas had managed to catch the army off guard. “That’s a good question,” he said.

Ismail Haniyeh, the exiled leader of Hamas, said that Palestinian fighters were “engaged in these historic moments in a heroic operation" to defend the Al-Aqsa Mosque in Jerusalem and the thousands of Palestinian prisoners held by Israel.

In the kibbutz of Nahal Oz, just 4 kilometers (2.5 miles) from the Gaza Strip, terrified residents who were huddled indoors said they could hear constant gunfire echoing off the buildings as firefights continued even hours after the initial attack.

“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,” said Mirjam Reijnen, a 42-year-old volunteer firefighter and mother of three in Nahal Oz.

Israel has built a massive fence along the Gaza border meant to prevent infiltrations. It goes deep underground and is equipped with cameras, high-tech sensors and sensitive listening technology.

The escalation comes after weeks of heightened tensions along Israel’s volatile border with Gaza, and heavy fighting in the Israeli-occupied West Bank.

Saturday's wide-ranging assault threatened to undermine Netanyahu’s reputation as a security expert who would do anything to protect Israel. It also raised questions about the cohesion of a security apparatus crucial to the stability of a country locked in low-intensity conflicts on multiple fronts and facing threats from Lebanon's Hezbollah militant group.

Hezbollah congratulated Hamas on Friday, praising the attack as a response to “Israeli crimes” and saying the militants had “divine backing." The group said its command in Lebanon was in contact with Hamas about the operation.

Israel has maintained a blockade over Gaza since Hamas seized control of the territory in 2007. The bitter enemies have fought four wars since then. There have also been numerous rounds of smaller fighting between Israel and Hamas and other smaller militant groups based in Gaza.

The blockade, which restricts the movement of people and goods in and out of Gaza, has devastated the territory's economy. Israel says the blockade is needed to keep militant groups from building up their arsenals. The Palestinians say the closure amounts to collective punishment.

The rocket fire comes during a period of heavy fighting in the West Bank, where nearly 200 Palestinians have been killed in Israeli military raids this year. In the volatile northern West Bank, scores of militants and residents poured into the streets in celebration at the news of the rocket barrages.

Israel says the raids are aimed at militants, but stone-throwing protesters and people uninvolved in the violence have also been killed. Palestinian attacks on Israeli targets have killed over 30 people.

The tensions have also spread to Gaza, where Hamas-linked activists held violent demonstrations along the Israeli border in recent weeks. Those demonstrations were halted in late September after international mediation.