SIDON, Lebanon (AP) _ Government troops and tanks advanced into southern Lebanon Monday in a bid to restore Beirut's authority, meeting scattered resistance from Palestinian guerrillas who refused to lay down their arms.

The troops spread out in the port of Sidon, 25 miles south of Beirut, and on several hilltops to the east. But the army made no attempt to enter sprawling Palestinian refugee camps, which also serve as bases for guerrillas.

The army reported two minor clashes with Palestinians and allied Sunni Muslim militias, and one army unit was blocked from taking up its position by Palestinian guerrillas. Two Sunnis were reported killed and six people were wounded.

The advance by 6,500 soldiers was the latest step in efforts by the Syrian- backed government of President Elias Hrawi to end Lebanon's 16-year-old civil war. The army already disarmed militias in the Beirut area under a 1989 accord that reformed the government's structure to give Muslims more equality with Christians.

Hrawi's success in re-establishing sovereignty in Sidon and other southern areas will be key to further steps, such as persuading Israel to withdraw from the border enclave in southern Lebanon it maintains to protect Israel from guerrilla attacks.

The Israelis have said they will not give up their ''security zone'' while Arab guerrillas are free to launch attacks on Israel and while Syria has 40,000 soldiers in Lebanon.

Failure to expand the government's control could mean a resumption of the civil war, which has left 150,000 killed and 200,000 wounded. It also could scare off thousands of Lebanese who have begun returning from abroad to help rebuild their shattered nation.

Hundreds of civilians fearful of potential violence fled the southern region Monday in cars loaded with mattresses and other household goods.

But others gathered on streets and cheered the army troops, sprinkling them with rice and rose water, the traditional Lebanese welcome.

The army began moving at dawn, and guerrillas of the Palestine Liberation Organization went on alert. An estimated 6,000 guerrillas are headquartered in the refugee camps of Ein el-Hilweh and Mieh Mieh on Sidon's flank and in hills to the east.

PLO leaders pledged not to block the army. But they also refused to give up their weapons, saying they needed them to defend thousands of refugees in Lebanese camps and to continue their struggle against Israel.

The PLO's security chief in southern Lebanon, Kamal Medhat, declared Sunday that the guerrillas would fight if the army tried to enter their camps.

Hrawi took an equally adamant position, broadcast by state-run Radio Lebanon: ''We will not allow any militias or foreign armies to maintain any influence within Lebanon.''

PLO gunners fired at least five mortar rounds toward army troops Monday, and two brief firefights were reported in the village of Salmieh, 2 miles east of Sidon.

Police said two Sunni Muslim militiamen were killed and five wounded in the exchanges. A Lebanese soldier was slightly wounded. There were no immediate reports of Palestinian casualties.

Police said 20 Palestinians and 12 Muslim militiamen were taken into custody for refusing to evacuate positions in Salmieh.

At sundown, army troops stationed on hillsides opened fire with machine guns. A senior military source, who insisted on anonymity, said the firing was aimed at ''clearing the terrain around our positions.''

The Palestinians did succeed in blocking one army unit by sending 500 fighters before dawn to the deserted village of Kfar Jarra, 3 miles east of Sidon.

''The army can proceed with its deployment, but we won't withdraw,'' said the PLO commander in Kfar Jarra, who identified himself only by the code name Abu Ali.

The move delayed the army's advance to a village farther east, which commands the terrain northeast of Sidon.