The heaviest shelling in weeks erupted across the Israeli-Lebanese border today, killing one Israeli and at least six Lebanese and raising fears of a major new explosion in the tense region.

Israeli Chief of Staff Mordechai Gur said Israel considered the six-week-old U.S.-arranged cease-fire in southern Lebanon broken and he said Israel was thinking about responding.

The collapse of the cease-fire, which took effect Sept. 26, raises the possibility of renewed fighting between Israeli and Palestinian forces, which could spread to Syrian troops present in Lebanon as peacekeeping units. Any major outbreak in the area could seriously hamper current efforts to resume negotiations on an overall Middle East peace settlement.

Lebanese Foreign Minister Fuad Butros met separately with the U.S. and Soviet ambassadors in Beirut today in an attempt to prevent the fighting from spreading. Soviet Ambassador Alexander Soldatov told reporters afterward, "My government believes the threat of an invasion of south Lebanon is very grave."

A State Department spokesman said in Washington that the United States had called on all sides to "show restraint" and he said U.S. officials were working to restore the cease-fire.

Israeli Defense Minister Ezer Weizman warned yesterday that Israel would respond "swiftly and fundamentally" if Palestinians continued to shell Israeli territory from sanctuaries in Lebanon.

There was no clear indication today whether a severe Israeli artillery bombardment that reached 12 miles into Lebanon constituted this response or whether Israel planned more drastic action, such as sending troops into Lebanon or attacking the Palestinian strongholds from the air.

In today's exchange of shelling, a dozen rockets fired from southern Lebanon landed in the northern Israeli resort town of Nahariya, killing a woman and injuring five persons.

Israeli artillery pounded several towns in southern Lebanon and, according to official reports from the area, killed six Lebanese civilians. Palestinian sources in Beirut said a three-hour Israeli barrage had killed 20 persons and wounded scores more.

Under the Sept. 26 cease-fire, Israeli forces were withdrawn from southern Lebanon, where they had been assisting rightist Lebanese troops opposed to the Palestinians. Palestinian commandos in the area were to be gradually reduced and Lebanese army units were to return to the area to maintain order. The simmering hestilities have prevented the lightly armed Lebanese army troops from carrying out their role, however.

Although there are those who would welcome an excuse to raid southern Lebanon, there are powerful constraints on Israel. The government must take into account that the United States is firmly committed to the territorial integrity of Lebanon and that negotiations concerning a Geneva peace conference are at a very delicate stage. The small raid into Lebanon in support of Christian forces last September brought swift and strong diplomatic pressure from Washington to withdraw.

Deputy Defense Minister Mordechai Zipori told the Israeli press on Sunday that the rocket attacks were an attempt on the part of hardline Palestine Liberation Organization forces to sabotage the cease-fire in southern Lebanon.

It is recognized that nothing could suit these PLO forces' more than to provoke a large-scale Israeli raid across the border.

Such a move would unite all Arabs in a hardline position against Israel while at the moment there is considerable disagreement among the Arabs on how to move toward a Geneva conference.

It has been the policy of this government to try and put the onus of not wanting peace upon the Arabs and a large-scale raid into Lebanon would work against the image of Israel's peaceful intentions.

The question is being asked here tonight, however, just how long can Israel sit back while Israelis are being killed?

Katyusha rockets are easy to move into position and easy to conceal and although Israeli counterfire can harass the Palestinians, it cannot prevent or suppress similar rocket attacks in the future.

According to the Israel army spokesman, the rockets landed today in four separate barrages: at noon, at 2:30 p.m., at 3:30 p.m. and at 3:40 p.m. Rivka Lupo, age 35, was killed in the second barrage.

In a separate development in Israel today, an Israeli Arab was killed and about 30 police and Arabs injured when a riot broke out int the northern Israeli town of Majd el-Crum over an attempt to tear down an illegally built house.

It is believed to be the first major rioting involving Israeli Arabs and police in a year and a half and is certain to rais anew charges that Israel is using unusual and brutal force to repress troubles that develop with its Arab citizens.

According to police, hundreds of townspeople attacked the bulldozers sent by the Ministry of Interior to tear down the house, one of thousands reportedly built without permits in northern Israeli areas heavily populated by Arabs. The government relented on many of the illegal structures, but about 200 were ordered demolished.

When the bulldozers were attacked today and the rioters began stoning passing cars on the highway linking Haifa to Safad, hundreds of police were sent to the town. when the rioters refused to disperse, police fired into the air and then at the ground, killing one Arab and wounding at least a dozen. More than a dozen police were injured by rocks.