Outgoing U.N. Ambassador Andrew Young, opening a new U.S. bid to halt the Lebanese civil war, yesterday sharply criticized Israel's military incursions into Lebanon as "wrong and unacceptable to my government."

Young's statement to a U.N. Security Council meeting on Lebanon reflected mounting U.S. concern that the fallout from the fighting, pitting Palestinians and Lebanese Moslems against Israeli-backed Christian militias, could endanger the fragile Egyptian-Israeli talks on Palestinian self-rule.

"We condemn the policy of artillery shelling and preemptive attacks on Lebanese towns, villages and refugee camps which Israel and the armed Lebanese groups Israel supports have followed in recent months," Young said.

"Let there be no doubt or ambiguity about this. We cannot and do not agree with Israel's military policies in Lebanon. They are wrong and unacceptable to my government. They are painfully at variance with the values which Israel has traditionally espoused."

Young was forced to resign as U.N. ambassador earlier this month after he violated policy instructions by secretly meeting with a representative of the Palestine Liberation Organization. He subsequently rebelled publicly against such aspects of the Carter administration's Mideast policy as refusing to deal with the PLO and not taking a firmer stance against Israel's attacks on Lebanon.

But, in yesterday's debate, he prefaced his remarks by stressing: "I speak with the full authority of the United States government."

He also had harsh words for those Palestinian guerrillas whose use of southern Lebanon as a base for terrorist raids into Israel triggered the Israeli retaliation. Young said:

"We condemn those who boast of the murder of an Israeli mother and her child, the attack on a bus filled with Israeli civilians, or the explosion of rockets and bombs in Israeli towns and cities."

Then, in a pointed reference to the controversy kicked off by his own dealings with the PLO and last week's Security Council debate on Palestinian rights, Young added:

"If there is a strengthened understanding in my country of the importance of assuring that the legitimate rights of the Palestinians are included in a comprehensive settlement -- and I believe there is -- then it is time for the Palestinian leadership to recognize that their objectives cannot be achieved through violence and terrorism."

Despite this gesture toward evenhandedness, the chief significance of Young's statement was that administration policymakers felt compelled to use it as a vehicle for the sternest language the United States has directed against Israeli activities in Lebanon in more than a year.

The administration has been trying to smooth out the disarray in its Mideast policy caused by the Young-PLO incident and by a subsequent internal wrangle involving President Carter's special Mideast peace negotiator, Robert S. Strauss.

Strauss had cast doubt on the wisdom of the United States trying to win Arab world support for the peace process by making a gesture toward the Palestinians in last week's U.N. debate. He also precipitated a noisy quarrel about whether he or Secretary of State Cyrus R. Vance had charge of the U.S. role in the peace talks between Egypt and Israel.

This disarray exacerbated the already badly strained relations between Washington and Israeli Prime Minister Menachem Begin's government.

As a result, as recently as Tuesday night administration policymakers were known to be debating whether their first priority in getting Middle East policy back on course should emphasize a conciliatory line toward Israel or risk new tensions with the Israelis by confronting the Lebanon situation head-on.

The language about Israel that finally emerged in Young's statement yesterday indicated the administration's decision that the Lebanon civil war has become too volatile a threat to Middle East stability to be ignored.

In outlining U.S. proposals for a solution, Young restated past calls for cooperation by all sides with the U.N. peacekeeping forces in southern Lebanon, an end to Israel's preemptive strikes into Lebanon and support of the Christian militias and the withdrawal of Palestinian forces in southern Lebanon beyond the U.N. area of operations.

State Department spokesman Hodding Carter said that, in pursuit of these goals, the United States will begin intensive diplomatic consultations with all countries that have a stake in the Lebanese fighting.

The idea, he said, is to seek concerted initiatives that might lead to a halt of the fighting. But he emphasized that, for the present, the effort would be conducted through diplomatic channels, and he cautioned against the idea that Strauss, who is scheduled to visit the Middle East in mid-September, or some other official might make a "major flying mission" to seek an accord over Lebanon.

The urgency with which the United States views the situation also was underscored by the fact that Vance, in one of his first acts after returning from vacation yesterday, met privately with Israeli Ambassador Ephraim Evron.

But, despite the stress being put on the Lebanon situation by Washington, diplomatic sources seemed pessimistic yesterday that the new U.S. campaign can arrest the deteriorating Lebanese situation.

Gen. Alexander Erskine of Ghana, commander of the U.N. peacekeeping force, warned yesterday that participating countries might withdraw their forces if they continue to come under attack from the contesting factions.

Erskine's warning drew a sharp response from Israel, which said all U.N. casualties were caused by "Palestinian terrorists."

There also were indications that the Israeli government, while it might modify its tactics in an effort to hold down civilian casualties, has no intention of abandoning its policy of "strike anywhere, anytime" in reprisal against terrorism orignating on the Lebanese side of the border.

That, in turn, could trigger new U.S.-Israeli tensions and set back U.S. hopes for prodding the Palestinian autonomy talks forward and resolving the still sensitive issue of what kind of force should supervise Israel's withdrawal from the Sinai peninsula under its peace treaty with Egypt.

In Tel Aviv, Israeli state radio reported last night that Israel has proposed a trilateral meeting with Egypt and the United States in Washington Sept. 17 and 18. It said this was the main subject of the Vance-Evron meeting, Reuter news service reported.

The State Department had no comment on the report.

On the other side, the PLO, which feels it won a big propaganda victory through the U.N. Palestinian debate and the Andrew Young incident, is now hoping to win further backing for its cause at the upcoming conference of non-aligned nations in Havana and the annual U.N. General Assembly meeting.

As a result, many diplomatic sources feel the Palestinians would be reluctant, at this point, to agree to a withdrawal from southern Lebanon or other actions that might be viewed as a lessening of their resolve to keep the pressure on Israel.