Nicaragua's new government formally took power today and immediately issued decrees expropriating all of former president Anastasio Somoza's property and dissolving the National Guard and Congress.

Tens of thousands of Nicaraguans gathered in the capital's central square to cheer the arrival of the fivemember Junta of National Reconstruction government from its provisional capital in Leon.

But while the junta members, some of whom had been little known to the majority of the people, were jubilantly received, the day belonged to the Sandinista National Liberation Front guerrilla who won the civil war.

Leaders whose names have become legend here -- Tomas Borge, Eden Pastora, brothers Daniel and Humberto Ortega, Jaime Wheelock -- where wildly greeted ass heroes. In full uniform of olive-green fatigues and berets, each carrying a rifle, the guerrillas headed a mile-long parade of hundreds of Sandinista troops in trucks and jeeps.

Arrival of the rebel chieftains and the junta, riding atop a fire engine, trigged a 10-minute salute of gunshots into the air.

Not only the Sandinistas have guns, however, and the problem of dealing with the profusion of privately held arms cast the only shadow on today's euphoria.

Throughout Managua's poor barrios, youthful members of the popular militias who helped Sandinista regulars, as well as numerous freelance bandits taking adventage of the chaos, celebrated the rebel victory by looting private homes, pillaging public buildings and commandeering automobiles at gunpoint.

Throughouy the city, members of the 5,000-member, well-disciplined Sandinista army maintained checkpoints and ordered all passing civilians to give up their guns.

The national radio issued repeated warnings that "the junta and the Sandinista command absolutely condemn the pillaging and looting of private property. It will be punished to the maximum of revolutionary justice.

"Destruction will be ended with the end of Somozaism," the announcements said. They called on nighborhood civil defense committees, formed during the war, to aid Sandinista soldiers in maintaining order.

Other announcements urged the militia youth to stop shooting into the air. "We understand your happiness," they said, "but this is not the way to show it. The people have won peace and they deserve to have it."

Despite earlier fears that the new government would have problems with renegade National Guard soldiers, who took flight as the Sandinistas entered the city yesterday morning, there as little indication that any of the trouble came from members of the previous government.

In fact, today's decree dissolving the National Guard seemed almost superfluous. After 46 years as Nicaragua's feared armed force, the backbone of the Somoza regime, the 12,000-member National Guard appeared to have disappeared overnight.

Leading commanders had fled the country en masses when the government of Anastasio Somoza and later when an interim administration collapsed. Guard troops discarded their uniforms and guns and took refugee in centers designated as part of the cease-fire agreement, or melted back into civilian life.

Although weeks of negotiations over the future role of the guard took place before the war ended, and the junta at least initially agreed to let some guardsmen join the new army, it is unclear hether any will attempt to do so.

The junta said today the Sandinistas will constitute Nicaragua's new armed force. Still to be determined is which of the former army's remaining officers and soldiers will be subjected to anticipated trials for "crimes against the people"

The Sandinistas hold a number of National Guard prisoners in outlying cities. Some have already been released, while what is at the present believed to be a handful of others judged particually notorious were summarily excetuted during the war.

Several thousand have taken refuge under the protection of the Red Cross, Catholic churches and Latin American embasies here.

Late this afternoon the national radio also issued an urgent call to those organizations to come to the Somoza government's National Security Center to help what it said were approximately 100 prisoners held in underground cells that the former government had left there.

While the junta's program remained vague, today's ininaugural and victory speeches contained little to sustain fears shared by the United States and conservative Nicaraguans that the elimination of vestiges of Somoza and the hard task ahead of reconstructing the devastated country.

"The real revolution is still ahead," junta member Alfonso Robelo said.

'It is going to be hard, but that is nothing to the people of Sandino," he said in reference to Augusto Cesar Sandino, a 1920s guerrilla from whom the Sandinistas take their name.

"Today we begin a new Nicaragua," junta member Sergio Ramirez said, "that all Nicaraguans made possible with their blood. Today we burry Somozaism forever." In announcing the decree expropriating Somoza's extensive land and business holdings, Ramirez said, "the land of Somoza will now be under the feet of the peasants of Nicaragua."

Sandinista lader and junta member Daniel Ortega told the crowd "the junta puts itself at your orderss. It is you who will decide our future."

Guerrilla leader and newly named Interior Minister Tomas Borge warned thay while "U.S. interventionism" in the civil war "was avoided, if it comes, the people of Sandino will again take up arms and fight." But even the specter of U.S. power was largely treated kindly today.

Special U.S. Ambassador William Bowdler, whoe handling of negotiations with the junta in Costa Rica before the fall of Somoza brought initial bitterness, rode in the parade along with the foreign ministers of the Dominican Republic and Panama and other Latin American Officals.

"I think it would have been better if things had gone according" to a U.S. plan to add more moderate members to the junta and preserve the National Guard, Bowler said in answer to a question. "But those plans have a way of not working out." CAPTION: Picture, Nicaragua's new ruling junta rides aboard a fire engine in the Managua victory parade to the central square. The five-member junta arrived earlier from Leon.