TEL AVIV, MAY 30 -- Israeli forces intercepted two speedboats filled with Palestinian fighters as they headed toward this city and nearby Mediterranean beaches this morning, killing four of the guerrillas, capturing 12 and preventing what military officials said was to have been a large-scale attack on civilians.

The first boat was intercepted by the navy 20 miles offshore from Tel Aviv after being spotted on radar at about 7 a.m., officials said, and its heavily armed crew of five was captured without casualties.

Following a two-hour chase, a force of navy boats, air force attack helicopters and army troops cornered the second speedboat with 11 Palestinians aboard on a beach near Nizzanim, 18 miles south of Tel Aviv. Four of the Palestinians were killed in a firefight that broke out only about a half-mile from a beach club where 3,000 Israelis were relaxing on a sunny national holiday.

The Palestine Liberation Front, a small, radical faction of the Palestine Liberation Organization led by Abul Abbas, said in Baghdad that its forces launched the attack to avenge the killing of seven Arabs on May 20 by an Israeli Jew described by authorities as deranged, Reuter reported. The PLF said the guerrillas intended to land and attack selected targets.

Acting Prime Minister Yitzhak Shamir and his aides promptly argued that the attack proved the PLO had not kept the December 1988 pledge by Chairman Yasser Arafat to renounce terrorism.

"An event like this comes along . . . and proves something that we have been claiming all along," Foreign Minister Moshe Arens said tonight. "The United States is holding talks with the PLO based on the assumption that the PLO has stopped engaging in terror. I hope this event will convince the U.S. administration."

In a press conference here, senior Israeli military officials said weapons and maps recovered from the boats and interrogation of the captured fighters showed that the force, which originally consisted of six boats, intended to attack hotels, beaches and other key points along the Tel Aviv shoreline.

"There is no doubt that the terrorists wanted to achieve mass killings in a spectacular fashion," said the army chief of staff, Lt. Gen. Dan Shomron.

Israeli officials speculated that the attack was timed to coincide with this week's Arab summit meeting in Baghdad.

The Palestine National Front, an Iraqi-backed faction of the PLO, has attempted a number of spectacular attacks against Israel, including the 1985 hijacking of the cruise ship Achille Lauro, in which an American passenger was killed. Washington unsuccessfully sought the extradition of Abbas from Italy and Yugoslavia following the incident.

Officials said tonight that PLF fighters had attempted to cross the Israeli border three previous times in the last 18 months, once from Jordan and twice from Lebanon.

Israeli commentators speculated that the latest attack could have the effect of relieving recent U.S. pressure on Israel. The Bush administration has shown open irritation with Shamir's refusal to go along with a plan for Israeli-Palestinian negotiations, and sharply criticized a government move to support Jewish settlement within Jerusalem's historically Christian Quarter.

Bush also called on Israel to show greater restraint after 16 Palestinians were killed in clashes with Israeli soldiers in the occupied West Bank and Gaza Strip last week. The violence, which followed the slaying by an Israeli civilian gunman of seven Palestinian workers in the Tel Aviv suburb of Rishon LeZion, prompted Washington to support the dispatch of a U.N. investigative team to the occupied territories, a measure Israel says it will not accept.

Asked on state radio if today's attack would "get Israel off the hook" in Washington, Arens replied: "We don't need to be let off the hook. We are an island of democracy and sanity here in the Middle East, though that's not always recognized by people around the world."

The commander of the navy, Rear Adm. Micha Ram, said today's attack was prepared by the PLF in Benghazi, Libya, 800 miles from Israel. Three days ago, Ram said, a merchant ship carrying the six attack boats sailed into the Mediterranean, and later released the speedboats about 120 miles off the Israeli coast.

Ram said that the merchant ship apparently headed toward Port Said, Egypt, after releasing the speedboats, and that Israeli officials notified Egypt. However, officials said tonight that they did not know where the ship was or whether Egypt had taken action.

Officials said the fiberglass speedboats, manufactured in Libya under a license from Cyprus, were powered with two 220-horsepower engines and armed with 23mm machine guns. They carried Katyusha rockets and light weapons, including submachine guns and rifles, officials said.

Ram said three of the Palestinian boats broke down during the voyage to Israel and a fourth, a fuel supply boat, sank. He said the Palestinian guerrillas gathered in the two remaining boats, which then separated off Tel Aviv, with one heading north and another south of the city.

The boat that headed to the north was captured by an Israeli patrol boat. The Palestinian fighters surrendered as the Israeli craft approached and were captured after being ordered by the Israelis to jump into the sea, officials said. Authorities then activated the coastal defense system, deploying special troops and sending out spotter planes to search for other attack boats.

The second was spotted soon afterward, but avoided capture for two hours, outrunning Israeli patrol boats and eluding the fire of attack helicopters, Ram said. Eventually the Palestinian craft neared Nizzanim, and some bathers told journalists later that it roared within 200 yards of the popular beach club. The club was packed for the holiday of Shavuot, which commemorates the handing down of the Ten Commandments. The bathers said the Palestinian fighters could have shot Israelis crowding the beach but did not fire.

Ram disputed this account, saying the speedboat did not pass within range of civilians on the beach and landed in an area more than a mile south of the beach club, where soldiers were waiting for it. Some of the Palestinians surrendered immediately while others fled toward nearby sand dunes, where four were killed in a burst of Israeli fire.