The group running the besieged Gaza Strip fired thousands of rockets as dozens of Hamas fighters infiltrated the heavily fortified border in several locations, catching the country off-guard on a major holiday. The surprise operation comes after thousands of Israeli settlers in recent days carried out provocative tours of the Al-Aqsa Mosque complex in occupied East Jerusalem during the Jewish holiday of Sukkot.

Six hours after the invasion began at daybreak on Saturday, Hamas fighters were still fighting gun battles inside several Israeli communities. Israel says at least 22 people have been killed and hundreds wounded.

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.

Prime Minister Benjamin Netanyahu declared Israel was “at war” and called for a mass mobilisation of army reserves. The invasion revived memories of the 1973 war practically 50 years to the day.