BEIRUT, Lebanon (AP) _ Several militias announced today they would join forces to prevent PLO chairman Yasser Arafat from re-establishing a power base in south Lebanon.

Sources close to Druse leader Walid Jumblatt said he was named commander of the new Joint Nationalist Deterrent Force and had instructed his lieutenants to select 2,500 recruits, who were to come from his Progressive Socialist Party, Nabih Berri's Shiite Amal militia and the pro-Syrian Baath Party.

''We reject any Palestinian expansion in Sidon or elsewhere in Lebanon,'' Jumblatt told a news conference at his west Beirut home. He said Arafat's return would ''expose Lebanon and Syria to new Israeli military adventures, which we don't accept.''

Israel has carried out nine air raids against refugee camps and guerrilla bases in the Sidon area this year.

Lebanese allies of Syria, which has been at odds with Arafat, decided to form the new force Tuesday and lined up behind Berri, whose militia has been fighting the Palestinian guerrillas.

A statement issued after a meeting of militia commanders at Berri's heavily fortified home accused Arafat of provoking clashes ''to set the stage for his return to south Lebanon.''

''His plan to return has entered a serious stage of implementation,'' the statement said.

Arafat's guerrillas were the dominant force in south Lebanon until driven out in Israel's 1982 invasion, and Amal has been attempting to prevent the Palestine Liberation Organization from reasserting its power in the predominantly Shiite south.

United Nations and Palestinian sources say an estimated 3,500 Palestinian guerrillas have filtered back to refugee camps in Beirut and the southern port cities of Sidon and Tyre in the past 16 months.

On Saturday, PLO forces staged their first major offensive in Lebanon since 1982 when they stormed out of refugee camps near Sidon and captured a string of hilltop villages from Amal fighters.

Tyre-based reporters said guerrilla defenders of the Rashidiyeh refugee camp traded sniper fire today with Amal militiamen.

Thirty-three Shiites and Palestinians have been killed and 180 wounded since Amal besieged Rashidiyeh on Oct. 1.

Creation of the new force marked a shift of militia alliances, as Jumblatt's fighters had aided Arafat when Amal tried to drive PLO forces out of refugee camps in Beirut last year.

In east Lebanon, the Syrian Army swapped two captured Shiite Moslem militiamen for four Syrian soldiers seized by the Iranian-backed Hezbollah, or Party of God, police reported today.

A police statement said the exchange occurred late Tuesday in the Bekaa Valley town of Mashgara, 22 miles southeast of Beirut.

Hezbollah gunmen abducted the four Syrian soldiers early Tuesday from a checkpoint at Mashgara to trade them for their two comrades.

Hezbollah wants to establish an Islamic republic in Lebanon. Syria maintains 25,000 troops in the Bekaa and northern Lebanon.