Egyptian President Hosni Mubarak, charging the United States with "a lack of firmness" in the conflict in Lebanon, appealed in a letter released today to Western European leaders to "play an important part" in restoring peace and independence to Lebanon and resolving the Palestinian problem.

But the leaders of the 10 nations of the European Community, here for a two-day summit conference, fell into dispute about how strong and specific a statement to make on the crisis. They faced a strong French effort to win support for a plan demanding Israeli forces move away from Beirut and the leadership of the Palestine Liberation Organization withdraw to four Lebanese camps.

The battle in Lebanon was a major topic of concern at the meeting, which also heard a fresh round of top-level criticism of the Reagan administration about a number of recent economic moves--including extension of sanctions against the Soviet gas pipeline to Western Europe--that threaten West European jobs.

Mubarak in an unusually direct appeal dated Saturday, criticized the United States for vetoing a U.N. Security Council resolution calling for Israeli withdrawal from Beirut and for voting against a General Assembly resolution demanding an Israeli withdrawal from Lebanon. He said the votes Saturday "proved that although the United States has accepted to play the role of full partner in the peace process, it is not yet willing to adopt the policy that would be consistent with this role."

"Lack of firmness on the part of the United States," Mubarak continued, "gives Israel the impression that it can count on an American backing regardless of its policy toward the Palestinian people." The Egyptian president urged the European leaders to "play an important part to ensure that the [Israeli] aggression would not be rewarded," that Lebanon is made independent and that a political solution is found for the Palestinian problem.

Spokesmen for the Europeans, meanwhile, reported the group was moving toward agreement on a statement to be released Tuesday that would attack the United States for a series of economic actions--involving the pipeline project bans, U.S. tariffs on European steel and agricultural imports and continued high U.S. interest rates. European officials said they regard these as calling into question understandings reached at the economic summit of the seven major industrialized nations at Versailles, France, earlier this month.

But while talking tough, senior officials, including French President Francois Mitterrand, indicated today that they hoped to resolve current conflicts through intensified bilateral and Common Market contacts that would avoid Europe engaging America in a full-scale trade war.

Opening the conference, Belgian Prime Minister Wilfried Martens, the current community chairman, suggested that U.S.-European differences over the situation in Lebanon were contributing to the image of the United States at odds with its allies and increasingly alone in the world.

Pointing to the 127-2 U.N. General Assembly vote Saturday, Martens said, "This event cruelly highlights Washington's isolation. It also underlines the opposition that exists between our countries and the U.S. at a time when . . . the relations between Washington and the community have been thrown into doubt by other grave questions."

The letter was apparently aimed to encourage backers of a European peace initiative in the Middle East, an aim pursued intermittently by the community since the Carter administration's Camp David initiative appeared to founder two years ago.

British and West German government spokesmen sounded doubtful today that the European leaders would use the Lebanese crisis to produce a statement that went beyond the principles laid down at the 1980 Venice summit. That statement called for a Middle East settlement that ensures Israel's security while providing for the right to self-determination of the Palestinians. It also said the Palestine Liberation Organization should be "associated" with any peace talks, a position that has been objectionable to Israel and the United States.

British officials attributed a delay in drafting any new European statement on the Middle East to reluctance to rush out a document with events changing so rapidly. But a Belgian source said the delay was the result of a rejection by France and Greece of a draft that had been long on general principles but short on a prescription for action to solve the conflict in Lebanon.

A second draft was reportedly being drawn up to incorporate a French proposal advocating that Israeli and PLO forces both withdraw from Beirut, that a demilitarized cordon around Beirut be established, and that PLO members and their leadership be placed in four camps in Lebanon.

Unclear from the piecemeal reports of the French plan available to reporters was what would be the fate of PLO weapons in the role the PLO would be expected to play in a future peace settlement for the region.