Barrages of rockets were fired from the blockaded Gaza Strip on Saturday, killing at least one person in Israel, an AFP journalist and medics said.
Rockets streamed across the sky repeatedly after the first launches from multiple locations across Gaza at 06:30 am (0330 GMT), the AFP journalist reported.
The Israeli army warned of sirens across the country's south and central areas, urging the public to stay near bomb shelters.
An AFP journalist in Jerusalem saw rockets being intercepted, moments after sirens blared across the city.
The armed wing of Palestinian group Hamas said it was behind the fire, claiming its militants had launched more than 5,000 rockets.
"We decided to put an end to all the crimes of the occupation (Israel), their time for rampaging without being held accountable is over," the group said.
"We announce Operation Al-Aqsa Flood and we fired, in the first strike of 20 minutes, more than 5,000 rockets."
Israel's military said Hamas was behind "a combined attack, including the firing of rockets and terrorist infiltrations into Israeli territory from the Gaza Strip."
A woman in her 60s was killed "due to a direct hit" in Israel, the Magen David Adom emergency services said.
Fifteen others were hurt, two of them seriously, medics said.
The office of Israeli Prime Minister Benjamin Netanyahu said the premier will shortly convene security chiefs over the violence.
- Earlier border protests -
Israel has imposed a crippling blockade on Gaza since 2007 after Hamas took power.
Palestinian militants and Israel have fought several devastating wars since.
The latest fire follows a period of heightened tensions in September, when Israel closed the border to Gazan workers for two weeks.
The shutdown of the crossing came as Palestinian protest rocked the heavily militarised border.
Protesters had resorted to burning tyres and throwing rocks and petrol bombs at Israeli troops, who have responded with tear gas and live bullets.
Critics had slammed the border closure as collective punishment against thousands of Palestinian workers, who have far greater earning potential in Israel than Gaza, where unemployment is rife.
Resuming their passage on September 28 had raised hopes of calming the situation in Gaza, home to 2.3 million people.
In May, an exchange of Israeli air strikes and Gaza rocket fire resulted in the deaths of 34 Palestinians and one Israeli.
So far this year at least 247 Palestinians, 32 Israelis and two foreigners have been killed in the conflict, including combatants and civilians on both sides, according to Israeli and Palestinian officials.
The vast majority of fatalities have occurred in the West Bank, which has been occupied by Israel since the 1967 Arab-Israeli conflict.
There has been a rise in army raids, Palestinian attacks targeting Israelis and Israeli settler violence against Palestinians and their property.
Several far-right Israeli ministers live in settlements in the West Bank, which are deemed illegal under international law.
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