GAZA CITY, Gaza Strip ? Israeli forces backed by tanks and helicopters moved deep into Gaza City early today, killing 12 Palestinians and wounding 51 in the bloodiest fighting in Gaza in months.

The large-scale assault came three days before an Israeli election that is expected to return Prime Minister Ariel Sharon to power.

Witnesses and security officials said about 35 Israeli tanks rolled into the city of 300,000 from three directions. They were backed by two helicopter gunships that fired machine guns at various targets, including gunmen in crowded streets.

The incursion was the deepest into Gaza City in more than two years of fighting. Tanks were seen a half a mile from Palestine Square, which marks the center of the city.

Palestinians responded by using mosque loudspeakers across the city to call gunmen into the streets to wage “holy war” against the Israelis.

The Israeli military said its forces came under heavy rifle and anti-tank fire, and soldiers returned the fire. Twelve Palestinians were killed and 51 wounded, eight critically, hospital officials said.

It was the bloodiest day in the Gaza Strip since August, when 13 people were killed in an Israeli attack on the southern city of Khan Younis. Casualties were being brought to Gaza’s Shifa Hospital in cars and ambulances.

Maj. Gen. Abdel Razek Majaidie, head of Palestinian security in Gaza, accused Sharon of “adding a new page to his dark record. This crime against innocent civilians is another part of his continuous war against the Palestinian people and Palestinian homeland.”

Sharon stood to benefit from an upsurge of violence as Tuesday’s elections approached.

Palestinians surround the remains of a destroyed bridge in Beit Hanoun in the Northern Gaza strip. Israeli forces early Saturday used dynamite to destroy the bridge, which they said was used by militants to fire rockets at Israeli towns.

His hard-line Likud party and its allies are ahead in the polls, and Sharon is expected to form a new government after the voting, but Likud leaders say they’re concerned that they might still not have enough electoral strength to form a stable ruling coalition.

The incursion began shortly after 10 p.m. Saturday. Palestinian legislator Ziad Abu Amr said he saw 10 tanks drive through his street. Another resident, Mohammed Tafesh, 29, said electricity was cut to the neighborhood.

The Israelis blew up the home of a Hamas militant and several metal workshops. The Israelis say the workshops are used to produce weapons.

Witnesses said a missile fired by an Israeli helicopter started a huge fire in an industrial area of eastern Gaza City.

The Al Aqsa Martyrs Brigade, affiliated with Palestinian leader Yasser Arafat’s Fatah movement, said its fighters blew up three Israeli tanks with explosives. The Israeli military denied the claim.

Also today, witnesses said nine Israeli tanks took up positions in the center of Beit Hanoun, a town at the northern edge of the Gaza Strip, where Israeli forces blew up four bridges a day earlier. Palestinians have been firing rockets and mortars from Beit Hanoun at Jewish settlements and nearby Israeli towns in recent days.