Israeli and Palestinian gunners exchanged artillery and rocket fire across Lebanon's southern border today as the United States began diplomatic efforts to halt the worst round of fighting between the guerrillas and Israel in three years.

The confirmed death toll from the Israeli bombing of downtown areas of Beirut yesterday neared 200 as more bodies were pulled from the wreckage of demolished apartment buildings. At least 700 persons were wounded in this and other attacks on Palestinian positions in the south, according to Palestinian and Lebanese government counts.

Beirut remained tense and jittery today as Israeli warplanes, apparently on reconnaissance missions, flew over in the morning and Palestinian antiaircraft guns fired on them.

In the wake of the devastating Israeli raid on Beirut, Palestinian guerrilla leaders swore revenge for what they called an "air massacre" and rained rockets on Israeli villages in upper Galilee, wounding at least six more persons.

The Palestine Liberation Organization called the rocketing a "down payment" on retaliation. A spokesman for the Marxist-oriented Democratic Front for the Liberation of Palestine, whose Beirut offices were badly damaged in the Israeli bombing, told reporters who visited the scene today:

"Of course, we are obliged to hit the Israelis. We are expecting a hot summer, a very hot summer."

In Israel, Deputy Defense Minister Mordechai Zippori said that while he welcomes a diplomatic effort by special U.S. envoy Philip C. Habib to seek a cease-fire, Israel "will continue to defend its citizens by any means necessary," even while Habib is in the region, Washington Post correspondent William Claiborne reported from Jerusalem.

[Amid a public debate in Israel over whether the massive bombing attack on PLO headquarters in Beirut would lower the risk of a crossborder war of attrition of heighten it, Zippori said other military options have not yet been exercised but may be if Palestinian guerrilla artillery attacks on Israel continue].

["There is no one solution to these problems, Zippori said. "We have to act in different ways and try to control their ability to launch any attacks. We did not until now use all the methods available. We have more solutions which in time we will use if we need to]."

[Habib arrived in Israel Saturday after meetings in Saudi Arabia on the Syrian missile crisis. He was scheduled to meet Sunday morning with Prime Minister Menachem Begin. President Reagan ordered Habib to Israel after the bombing of Beirut in what Washington described as "the first step" in an effort to arrange a cease-fire].

[Although Zippori said on Israeli radio that destruction of bridges spanning Lebanon's Litani and Zahrani rivers will impede the flow of Palestinian artillery to within range of Israel, the value of air strikes was questioned by prominent Israeli military figures].

[Former Army chief of staff Mordechai Gur said the guerrillas could easily transport artillery ordnance across the river, by mules and donkeys if necessary, and that the air strikes would not put an end to shelling of Israel's northern settlements.]

There were conflicting reports in Beirut about the military situation in southern Lebanon, but a U.N. spokesman said Israeli artillery pounded the Palestinian refugee camp of Rashediyeh near Tyre for five hours today. Other reports said one person was killed and three were wounded in an artillery attack on Sidon, 25 miles south of Beirut. It was not clear whether the shelling came from Israeli-backed Lebanese secessinists led by Maj. Saad Haddad or from an Israeli gunboat operating offshore.

There were also unconfirmed reports that another bridge was destroyed between Sidon and Nabatiyeh. It would be the 10th in the last eight days of Israeli raids on roads and other communication links in southern Lebanon in a bid to isolate Palestinian positions there. Israel denied that its warplanes had gone into action today, leading to speculation here that the bridge was destroyed by artillery fire from Haddad's forces or an Iraeli gunboat.

Rescue workers, meanwhile, continued pulling bodies from the rubble of at least six demolished apartment buildings in the densely populated Fakehani section of Beirut hit in yesterday's Israeli raid.

The neighborhood houses offices and headquarters of several Palestinian groups, including PLO chief Yasser Arafat's Fatah, that were a prime target of the Israelis. While one building housing some offices of the Democratic Front and another housing those of the pro-Iraqi Arab Liberation Army were mostly destroyed, Fatah officials insited none of theirs was hit.

Bulldozers and cranes worked throughout the night and today to pull away the debris and unearth more bodies believed buried in the wreckage. A Democratic Front spokesman said his group lost 19 persons in the bombing and that the vast majority of the dead were women and children of Palestinian and Lebanese families living in the buildings. Beirut newspapers were filled with the pictures of wounded victims, many of them children.

The bombed area, which covers parts of two streets, was evacuated of civilians, as was much of the entire Fakehani and adjacent quarters, because of fears of more Israeli attacks. When Israeli jets passed over the city this morning, a group of visiting reporters dove for cover in basements and behind sandbags.

The Lebanese leftist National Movement, which is closely allied with the PLO, called on Arab countries to provide Lebanon with Soviet-made SA6 antiaircraft missiles to defend against Israeli raids.