The Israeli-backed militia of renegade Lebanese Army major Saad Haddad established a new garrison today in the southern port city of Sidon, the scene of several killings of Palestinians during the past two weeks.

At a press conference in Sidon, Haddad deplored the killings and said he was establishing the garrison in a former hospital because he wanted to help the Lebanese government spread its authority over the whole of Lebanon and avoid the country's partition, according to reports reaching here.

After the press conference, Haddad's forces paraded through Sidon, Lebanon's third largest city, before moving into their new barracks.

The establishment of the new garrison by Haddad, who is considered unlikely to act without the blessings of his Israeli backers, only deepened the mystery about Israel's intentions for the area. Haddad's militia of about 1,500 men is entirely dependent on Israel for weapons and supplies. His militia has controlled a strip of southern Lebanon along the frontier with Israel since the 1978 Israeli invasion.

After the most recent Israeli invasion last June, Haddad, a Maronite Christian, has made several unsuccessful attempts to extend his influence over the entire 25-mile-wide band of Israeli-occupied southern Lebanon that Israel says it wants as a security zone to prevent the return of Palestinian guerrillas.

Since then, western diplomatic and military observers here watched as the occupying Israeli Army attempted, in the words of one, to enthrone Haddad as the "lord of the south."

But they said Haddad has had great difficulty in recruiting to build up his militia, especially in the villages of Shiite Moslems, the most populous group in the south, where resistance to Haddad has been strong. In the minority Christian areas outside Haddad's frontier enclave, young men have tended to enlist in the militia of Haddad's rival, the Phalange's fiercely anti-Palestinian Lebanese Forces militia, which is also attempting to establish control in the south.

Two weeks ago Haddad dismantled some of his scattered checkpoints around the Sidon area and there followed a wave of killings of Palestinians in an apparent campaign to drive the refugees from Sidon and its suburbs. When some Palestinians fled, their apartments were frequently seized by the Lebanese Forces.

Many Palestinians have found themselves during the past two weeks in the position of begging for the reestablishment of checkpoints by Haddad's soldiers, a force they said they dislike because of its ties to Israel but one they fear less than the Lebanese Forces, which also has close ties with Israel.

Today, Haddad claimed that his "Free Lebanon" enclave now extends north from the Israeli frontier to the Awali River bridge north of Sidon, east to the Bekaa Valley town of Jubb Jannin and southeast to the slopes of Mount Hermon. That area would be roughly one-fourth of Lebanon and comprise much of the region Israel demands as its security zone.

In the negotiations for the withdrawal of its soldiers, Israel has made clear that one of its conditions is continued control over the area. Its negotiators also have said that Israel wants to keep a residual contingent of troops in the area to maintain permanent Israeli security installations.

In the long-stalled negotiations, Israeli, Lebanese and U.S. officials held their 15th meeting today in the Beirut suburb of Khaldah. According to an official statement issued after the session, drafting subcommittees met to work on language covering the withdrawal of troops and to discuss the ending of the state of war between Israel and Lebanon. The negotiators are scheduled to meet again on Thursday in the Israeli resort of Netanya, north of Tel Aviv.

Lebanese President Amin Gemayel's Cabinet today granted the Lebanese Army extraordinary powers to arrest people threatening national security and prepared for a long-delayed attempt to take control of East Beirut, United Press International reported. The decree, approved as 4,000 Lebanese troops were poised to enter the Christian-held area, gave the Army commander the right to try people threatening national security before a military court.

As of midnight, however, the troops had failed to move out of their barracks