A cease-fire mediated by the United States took effect in south Lebanon yesterday, quieting the guns of Palestinian guerrillas and Christian rightists and precipitating withdrawal from Lebanon of Israeli tanks and troops which had fought on the side of the Christians.

Heavy artillery and mortar exchanges echoed right up to the mid-morning deadline through Lebanon's southern mountains where Christians and Palestinians have been fighting sine the end of the Lebanese civil war last November. The fighting has been particularly intensive since Israeli forces crossed into Lebanon eleven days ago to help the beleagueved Christians.

The American ambassador in Beirut. Richard Parker , who has helped arrange the agreement, said the truce was "being observed by all sides." Israel however, reported a rocket attack on three Israeli villages shortly after the cease-fire went into effect.

The truce agreement amounted to an unprecedented - although indirect agreement between Israel and the Palestinian Liberation Organization.

It involved directly Lebanon, Syria and Israel, who agreed through the American intermediaries on cease-fire terms. Since Palestinian guerrillas were involved in the fighting, the PLO was drawn in as well. Israeli defense officials insisted that the accord was one between Israel and Lebanon, but acknowledged that "the PLO has given certain assurances" to the parties concerned.

The agreement calls for withdrawal of Israeli forces from Lebanon and a pullback from the borders area by palestinian guerrillas who will be replaced by a 1,470-man buffer force of the Lebanese army.

Israeli officials said they were highly satisfied with the terms of the agreement, which they maintained meant that the Palestinians will be required to withdraw from the border area from which they have been able to shell Israeli villages.

An associated Press reporter reporting from the Palestinian held border village of Khiam said there were no signs of guerrilla preparations to pull out from the area, as provided by the truce accord. He quoted Khiam's guerrilla commander as saying, "I have had only instructions to observe cease-fire."

The same reporter also reported that two Israeli tanks have remained in position between Khiam and Marjoun, one mile to the north, which is held by the Christians.

Israel had been providing artillery support and material assistance for the Christians for months, but on Sept. 16 they sent Israeli tanks into Lebanon trying to disloge Palestinian guerrillas from several strategic hill-top positions, including the villages of Khiam, and Ebelesaqi, and the Beaufort Castle, and edifice dating back to the Crusade period.

Until Sunday, however, Israel censored all reports mentioning the presence of Israeli forces in Lebanon. Yesterday, Israeli authorities allowed publication of such reports but prohibited all mention of their numbers or their placements.

Diplomats in the area suggested that it would take several days before it will be known whether truce has taken effect.

After its announcement in Beirut, a spokesman for the radical Popular Front for the Liberation of Palestine, whose leader is George Habash, said the group "shall allow no one to prevent us from striking at the Israeli enemy or existing in areas where we can strike from. We shall abandon none of our positions in southern Lebanon."

Habash's group pulled out of the PLO's executive council in 1974 because of what Habash considered to be moves toward a compromise settlement with Israel.

As the long row of Israeli tanks and armored personnel carriers rolled back across the northern border early this morning. Israeli officials made no effort to hide their satisfaction over the terms of the cease-fire.

The Israelis invited the world press to watch their soldiers waving good-bye to their Lebanese comrades-in-arms as they crossed into Israel near the northernmost village, of Metullah. The Christian fighters who came to see them off showered praise on Israel and the Israeli soldiers into the microphones of the assembled newsmen.

Just half an hour before the ceasefire came into effect, at 10 a.m. local time, two northern Israeli towns, Qiryat Shemona and Safed, were again hit by salvos of Katyusha rockets, which have been fired from across the border almost daily for the past five days. This time there were seven wounded. One of them, a little boy, was seriously hurt.

The cease-fire brought to an end, at least temporarily, a debate within Israel over strategy in southern Lebanon.

Lebanese government officials welcomed the truce as a sign that the "Shtaura accord" - a peace plan negotiated two months ago by the Lebanese. Palestinians and Syrians at a town east of Beirut - could now be implemented. But the provisions of that agreement permitted the Palestinian guerrillas to keep up to 300 men in the disputed border area.

Israeli commentators pointed out that the U.S mediated truce and two advantages for Israel. It not only calls for the pullback of all PLO units to points 6 miles away from the border, but also prohibits the PLO from firing from their new positions into Israeli territory. Moreover, yesterday's truce also recognizes Israel as a party to the settlement in southern Lebanon.

If the cease-fire holds, the Israel's are convinced that this will lead to a more permanent settlement on their northern border.

The Israelis were reported satisfied that the newly formed Lebanese army will move into southern Lebanon, perhaps even as early as next week, to protect Christian villages.