A 71-year-old Army officer, who helped orchestrate the tribally motivated coup that on Saturday toppled the government of Uganda, today was sworn in as the country's head of state.

Lt. Gen. Tito Okello, dressed in a camouflage uniform, took the oath of office at 3 p.m. on the steps of the Ugandan parliament building in Kampala and promised in a speech broadcast by Radio Uganda that "within a year Ugandans will have a country of their own choice."

Radio Uganda said earlier that elections would be held within 12 months in the East African country.

Following two days of looting that began shortly after military trucks rolled into the Ugandan capital Saturday morning, Kampala was quieter this afternoon, although residents still reported scattered gunfire. Most downtown shops were stripped of their goods, some United Nations offices were ransacked and city streets were littered with broken glass and discarded packing boxes, according to news agency and radio reports from the city.

A 6 a.m.-to-6 p.m. curfew remains in effect, and the country's border's remained closed. All flights into the country have been canceled. At Entebbe Airport near the capital, a man who answered the phone in the control tower today told a reporter who telephoned from Nairobi, "If you come here, you will be shot."

The new military government, had used state radio yesterday to introduce another military officer whose last name is also Okello, as the country's "leader." Today the government said that an executive prime minister would be appointed and that he would appoint a Cabinet.

Only one minister in the former government, Chris Rawkasisi, head of security in the president's office, was reported to be under arrest. Government employes, including heads of departments, were told to report to work on Tuesday.

The deposed president, Milton Obote, 60, is now in western Kenya, according to western diplomats in Nairobi. Diplomats said Obote, who fled Kampala by car the night before the coup, was in seclusion near the city of Nakuru at Kabarak, the private farm of Kenyan President Daniel arap Moi.

Uganda's new leader, who delivered a speech on Radio Uganda last night before it was clear that he would take control of the country, called Obote a "dirty man" who "has damaged the name of Ugandan citizens."

The U.S. State Department charged last year that about 200,000 Ugandans were killed or disappeared during the five years of Obote's second period in office. He was previously deposed by Idi Amin in 1971.

Amnesty International recently issued a report documenting what it called "widespread and systematic use of torture against detainees." Ugandan Army soldiers, who are ill-paid and undisciplined, have been blamed for the violence.

In the weeks leading up to Saturday's coup, there had been escalating tribal tension within the Ugandan Army between soldiers from the northern Acholi and Langi tribes. Western diplomats said today that Tito Okello, an Acholi, used this tribal rivalry, which had been festering for years within the Army, to seize power.

According to the diplomats, Acholi soldiers, traditionally the backbone of the Army, have been increasingly restive and ill-disciplined since early last fall when Obote, a Langi, appointed another Langi, Smith Opon Acak, to be Army chief of staff. Since then, Acholi soldiers had complained that members of their tribe were being disproportionately wounded and killed in front-line fighting against rebels in western Uganda, while Langi soldiers were receiving more than their share of promotions.

Tito Okello, a longtime ally of Obote's, reportedly wanted the chief of staff job or, at the very least, wanted the job to go to an Acholi, diplomats say. The appointment of Opon Acak is believed to have caused a rift in Okello's relationship with the president. The rift turned two weeks ago into armed conflict between Acholi and Langi soldiers.

Early last week, in what Okello described last night as "a safari," the general left Kampala with about 20 military trucks and traveled north toward the town of Gulu. Diplomats say that on the way, Okello stopped off at Army garrisons to round up sympathetic Acholis.

At about the same time, another Acholi officer, Brig. Basilio Olara Okello (no relation to the new leader), led a mutiny of his troops and began moving south toward Kampala. While Basilio Okello led the soldiers who seized Kampala on Saturday, several western diplomats believe that Tito Okello orchestrated the coup.

Uganda became infamous for high-level corruption and violence beginning with Amin's rule. But Tito Okello, the most senior official in the Obote government known to be involved in the new military regime, enjoys a reputation for integrity.

A career military man, he enlisted as a private in the King's African Rifles in 1940. In 1962, when Uganda became independent from Britain, he became a lieutenant in the new Army and within eight years had become chief of staff. Okello did not last last long in that job, however, and he was serving as commander of 2,000 troops in southern Uganda in 1971 when Amin seized power from Obote.

From exile in Tanzania, the two worked to oust Amin. A year later Okello led a band of 1,000 ill-trained and ill-equipped Ugandans on an abortive two-day invasion of their homeland.

After returning to Uganda in 1979 in a successful military operation (which he did not lead) that drove Amin into his own exile, Okello served as Army commander under Obote.

In his radio speech last night, Okello reassured his compatriots, saying, "The military coup is not affecting you, as you are citizens . . . . I would like you to keep your confidence in your country."