In the dusty, ramshackle Palestinian camp by the sea here, a sweating, shoving crowd roared and laughed today as Yasser Arafat delighted them with the memory of what had happened to all their foes in last summer's agonizing defeat in Beirut.

"Where is Begin? Where is Sharon? Where is Haig?" the Palestine Liberation Organization chairman asked. Then, "Where is the Palestinian revolution?"

The old men and women, the sinewy fighters in their new green fatigues, the little boys and girls in battle dress and standing in military formation, screamed in unison, "We are here."

Arafat smiled at them. "The winds did not shake the mountain," he said.

Evacuated from Beirut during the Israeli siege a year ago, expelled by Syria from Damascus and this Syrian-held area of Lebanon three months ago, Arafat was back, receiving a triumphal welcome from the camp refugees who were in the midst of the annual Moslem "feast of sacrifice" celebration and also the painful memory of the massacres of Palestinians at the Sabra and Shatila refugee camps in Beirut a year ago.

Arafat was coy about how he had managed to get back into Lebanon, but he disdainfully shrugged off speculation that he had sailed from Cyprus as one of his aides did last spring, stowed away in the hull of a smuggler's ship. The PLO chairman savored the mystery of his return and would not tell.

"You know the meaning of his arrival?" his trusted deputy Abu Jihad whispered confidentially. "He is telling the Syrians that 'if you are closing the door in my face, I'm coming through the window.' "

But Arafat's surprise arrival here two days ago appeared to be more complex than that. One thing he stressed repeatedly to reporters was his authority as commander of the joint Palestinian and Lebanese leftist forces that he said was never canceled despite his expulsion from Beirut.

He also said he no longer felt bound to honor his pledge not to return to Beirut because the Americans, the French and the Italians in the multinational peace-keeping force had not lived up to their end of the bargain and had left the refugees at Sabra and Shatila to be massacred. Arafat suggested, however, that he would probably wait for an "invitation" before returning to the Lebanese capital.

The comments were significant because of the continuing battles of Druze, one of the Lebanese leftist forces, against the Lebanese Army in the bloody mountains southeast of Beirut.

"We are in one trench with the antigovernment Lebanese national forces," Arafat, referring to the leftist factions, told the crowd at the refugee camp here today. "We will continue together until victory. This is our faith, this is our oath. This is a long march. We are fighters."

Lebanese and U.S. officials allege that his forces and the Syrian-backed PLO fighters in mutiny against him are responsible for the brunt of fighting in the Chouf mountains overlooking Beirut. Almost daily, Lebanese officials and press paint a nightmare of the return of PLO forces to regain control of Beirut.

Smiling mischievously, Arafat and his lieutenants acknowledged that some of their forces were fighting in the mountains, but they refused to reveal numbers or their actual role.

"We are there with small elements only," Arafat said. "The main forces are Lebanese forces. It is only those PLO units who are participating in the war of attrition against the Israelis."

Spokesmen for the Druze Progressive Socialist Party militia in Beirut said tonight that until they were recalled two days ago, only 150 PLO rebels had taken part in the mountain battles, but they angrily denied that forces loyal to Arafat were involved.

"Mr. Arafat is saying these things just to complicate our situation and we ask Mr. Arafat not to complicate our internal problems," a Druze spokesman said. "We assure you there are only Druze people fighting."

The truth appears to be somewhere in between. Arafat and PLO aides claim their fighters had been in the Chouf even before the civil warfare began in the aftermath of the Israeli withdrawal from the mountains. Although Arafat was vague about it, aides indicated that others moved in after the first big Druze battle with Christian Phalangists.

The PLO aides acknowledged, as the Lebanese have alleged, there there are both PLO loyalists and rebel units near the strategic heights overlooking Beirut in the town of Suq al Gharb, scene of the last few days' fierce fighting.

Just how effective they have been is unclear. Participation on the same side in Lebanon's civil war has not healed the split inside the PLO.

Arafat aides complained bitterly over the last two days that Syrian soldiers and the Syrian-backed followers of PLO rebel leader Abu Musa had prevented loyalist troops from reaching the battlefront, but they said they had often managed to make it through checkpoints and obstacles by "our special ways."

But the skirmishing between the opposing factions apparently continues, even on the Lebanese battlefield. Arafat aides said they had sent a fleet of their ambulances to rescue Druze casualties in the early stage of the Druze conflict with the Phalangists only to have some of them stolen by PLO rebels.

Loyalists here snickered about the fighting ability of the rebel forces. They alleged that when the Lebanese Army began its advance to secure its hold on Suq al Gharb and nearby areas, it easily rolled over the rebel PLO positions but was not able to overtake the Druze or the Arafat loyalists, who were in a different sector.

Arafat also expressed contempt for the rebels today. When asked if they were participating in the mountain fighting, he said, "You can ask the Syrians. They belong in the Syrian intelligence service. . . . They are not Palestinians."

Druze spokesmen say their allies have been the rebels and that has upset Arafat.

Arafat aides were careful about Druze sensibilities. Abu Jihad repeatedly stressed that while they were involved, the Druze were the ones leading the battles. But there was no question that the PLO wanted a role, despite the concern of some that fighting in the mountains might draw them into another battle with Israeli forces on Lebanese soil.

Lined up in formation at the camp today were a number of the fighters evacuated from Beirut last summer. Abu Jihad said they had been brought back from Tunis, North and South Yemen and the other farflung outposts where they were sent.

Abu Jihad also seemed to want to be in the battle in the mountains. Speaking alone with reporters this afternoon he shook with anger as he recalled the Sabra and Shatila massacres, the arrests of Palestinians by Lebanese authorities that followed and the bombings of Palestinian-owned shops in Beirut later on.

Participation in the current battles, he said, offered the hope for a more sympathetic government in Lebanon.

But later in the day, Arafat, speaking with reporters, appeared to shun involvement in the thorny Lebanese troubles. The Palestinian fighters are in the mountains, he said, because "they are following the Israelis. It is through this successful war of attrition that the Israelis are obliged to retreat."