Israeli-backed Christian militia shelled the U.N. peacekeeping force headquarters in southern Lebanon this morning after the commander of the rightwing militia declared what he called an independent "Free Lebanin" yesterday.

A U.N. spokesman in New York said the peacekeeping force's headquarters in Naqoura was "under very heavy motar and artillery fire," estimated at about 300 rounds. There was no report of casualties.

The spokesman said the U.N. force was still under orders not to fire back at the Christians, angered by the arrival in southern Lebanon of a unit of the Lebanese Army, which also came under their fire. The declaration of "independence" came at a news conference in northern Israeli town of Metullah:

Maj. Saad Haddad said the decision to declare a separate southern Lebanon was taken at an earlier meeting yesterday in Marjayoun of representatives of the 100,000 Lebanese living south of the Litani River.

Haddad had declared Tuesday that he would proclaim a republic if Lebanese Army troop moved into the U.N. policed area in southern Lebanon. His 1,500 troops control a six-mile strip on the border.

Christians have allied with the Israelis in a joint effort to keep Palestinian guerrillas out of the zone near the Israeli border and the Israeli as well as Haddad feel the Lebanese Army-rebuilding after its destruction in the 1975-76 civil war - will not control the Palestinians

Haddad has been disavowed by the Lebanese government in Beirut as a renegade. He called for the resignation of Lebanese President Elias Sarkis and said the "independent area" would return to Beirut's authority only after all Palestinian and Syrian troops left the country.

The Beirut government asked the world to ignore the declaration.

In Washington, a State Department spokesman declared U.S. opposition to the move.

While the Army units moved in east of the U.N. troops, Israel's military command reported that several rockets were fired "from Lebanese soil" into the western Galilee region, injuring one person and damaging several building. Israel returned the fire. CAPTION: Map, no caption, By Dave Cook - The Washington Post; Picture, Lebanese Christian militia commander Saad Haddad.