In a sharp escalation of its grinding war of attrition in Lebanon, Israel bombed bridges along the country's coastal highway today and hit two power stations in Beirut, plunging the center of the sprawling seaside capital into darkness after an exchange of shelling with Islamic guerrillas along the border.

Lebanon's Hezbollah, or Party of God, launched four barrages of rockets at northern Israel, killing two Israeli civilians and sending thousands to bomb shelters. Two Lebanese were killed in the bridge bombings, and, as the Jamhour power station on a hill overlooking Beirut spewed smoke and flames, four Lebanese firefighters were either wounded or killed as Israeli warplanes continued to pummel the facility, news agencies reported.

The attacks, which went tit-for-tat through the day and into the night, were the most serious between Israel and the Lebanese guerrillas in three years. They represented a sudden intensification of a conflict largely confined to the vicinity of the border since Israel's large-scale attacks in southern Lebanon during Operation Grapes of Wrath in 1996.

The flare-up seemed likely to cloud recent optimism generated by reports that President Hafez Assad of Syria and Israel's Prime minister-elect, Ehud Barak, are determined to resume negotiations on a peace agreement that would settle the long poisonous Lebanese-Israeli border conflict as well.

The strike on the Jamhour station, Beirut's main power plant, was apparently ordered by outgoing Prime Minister Binyamin Netanyahu, who remains in office despite his electoral defeat last month while Barak tries to form a coalition government. Barak was informed of the attack after the fact and was not consulted about the decision, officials said.

Netanyahu acted after a volley of about two dozen Katyusha rockets and mortar shells fired by Hezbollah guerrillas hit northern Israel this afternoon. Netanyahu, who has been almost absent from public life since his defeat May 17, said Hezbollah is "mistaken" if it intends to accelerate attacks on Israel in the interregnum before Barak takes office.

The Iranian-backed Hezbollah said in Beirut that it fired the first rocket volley to avenge Israel's "targeting of civilians in southern Lebanon," where a 40-year-old woman was wounded hours earlier when shellfire hit a village. The shells were apparently fired by the South Lebanon Army, an Israeli-sponsored force in the border hills. Subsequent volleys of rockets were launched in retaliation for Israel's series of air attacks in Beirut, along the coastal highway and in the Bekaa Valley to the east.

"In these situations, we must respond in order to defend our deterrent ability against Hezbollah," said Israel's military chief of staff, Shaul Mofaz. He said Israel had displayed great restraint until now but insisted that Hezbollah's "massive attack" on Israeli border towns and villages was a disproportionate act that demanded tough Israeli retaliation.

Israel, which has occupied a swath of southern Lebanon for two decades, has been mired in what it acknowledges to be an unwinnable war against Hezbollah. Mindful that many Israelis are fed up with the conflict, Barak promised during his election campaign to withdraw troops from Lebanon within a year.

On both sides of the border, the attacks caught residents off guard. Beirut, which is rebuilding from the civil war of the 1970s and the Israeli invasion of 1982, has enjoyed relative calm -- until tonight. "What is Israel doing to us?" said a stunned Beiruti, reached by phone this evening.

She said the lights went out suddenly in much of the center of town, leaving anyone without a generator fumbling in the dark. However, many restaurants and night clubs continued to operate as usual with the help of generators.

In Kiryat Shemona, Israel's main town in the north, angry residents accused the government of doing too little to retaliate. Some screamed at police officers who were urging them to take cover in bomb shelters. "We cannot continue to be Hezbollah's hostages," one man told Israeli television. "We are asking the government to respond."

CAPTION: Rescue workers in Lebanon transport a man wounded in the Israeli bombing to a hospital in Beirut. The raid followed a Hezbollah attack on a border area.