ROME, JAN. 28 -- Somali rebel leaders announced formation of an interim coalition government for their war-ravaged East African nation today, just hours after a guerrilla offensive reportedly toppled the 21-year authoritarian regime of President Mohamed Siad Barre and sent him fleeing into the countryside.

At a press conference here, a spokesman for the largest rebel group, the United Somali Congress, said the provisional government would seek to organize U.N.-monitored democratic elections as soon as order is restored in the country, and he pledged that Siad Barre would have a public trial once he is taken into custody.

Siad Barre and an unknown number of loyalist troops were forced from the presidential palace and other strongholds in the Somali capital, Mogadishu, by a weekend rebel offensive that apparently capped a month-long civil war. Thousands of civilians were killed in the weeks of fighting and tens of thousands fled the former Italian colony.

Rebel spokesmen here said they had spoken with guerrilla commander Gen. Mohammed Nur Galal in Mogadishu this afternoon and that Galal had told them Siad Barre was fleeing overland south towards the Kenyan border, where he may attempt to seek political asylum.

Abdirahim Mohammed Mohamud, a member of the rebel coalition's Rome-based executive committee, said guerrilla forces had been sent to intercept him at the town of Kismayo, about 300 miles south of Mogadishu, and that "he has no possibility" of escaping. "We know he was heading for Kismayo," said Mohamud, "and our soldiers got there first."

The rebel forces, an alliance of several main Somali clans which have long complained of widespread human-rights abuses by the Siad Barre government, launched full-scale hostilities last Dec. 30 with an assault on Mogadishu that settled into weeks of intense street fighting.

The final rebel push began Saturday and drove Siad Barre and his remaining defenders from his fortified palace to an enclave surrounding the international airport, rebel leaders said. After a brief resistance there, the president fled south, they said.

Rebel spokemen here said the new government's first priority will be to restore law and order in the country and to start rebuilding the capital, which has been devastated by shelling and looting. Earlier this month, most foreign embassies closed down as armed gangs took to the streets, raping and robbing. The last remaining aid agencies pulled out after three doctors from one relief group were wounded and their ambulances and equipment stolen by government soldiers.

By this afternoon, the airport was reportedly open again and a medical team from the international aid group Doctors Without Borders had arrived in Mogadishu to begin counting the dead and making arrangements for disposal of bodies, many of which were still lying in the streets. Rebel spokesmen here said there is no drinking water or electricity in the city and that they feared outbreaks of cholera and typhoid fever. Overseas help of all kinds is urgently needed, they said.

The provisional government formed today will consist of representatives from six Somali opposition groups, five of which took part in the conflict against the Siad Barre government. The sixth component, the so-called Manifesto Group, is composed of intellectuals and religious elders who sought a peaceful end to the regime.

"The provisional government will organize a popular referendum, which will be the precursor to political elections in the country," said rebel coalition foreign-affairs spokesman Abdulkadir Mohammed Abdulle. "Then we will see the reestablishment of the multi-party democratic system that all we Somalians in the opposition have always fought for."

Siad Barre came to power in a military coup in 1969, and during his years in power he steered the predominantly Moslem nation through shifting diplomatic associations with the Soviet Union and the United States, becoming Washington's closest ally in the Horn of Africa during most of the 1980s.

The United States has sharply curtailed that relationship in recent years, however, in response to continuing allegations of government human-rights violations and a lessening of the strategic importance of the region.