Retired Army general Reynaldo Bignone, sworn in as military president here today, promised to unify Argentina's divided armed forces behind a government he said would not offer "great changes" during a planned 18-month transition to democracy.

Bignone was appointed president by the Army last week. His government has clearly emerged as a conservative movement that would seek to brake the major shifts in foreign and domestic policies sought by some civilian and military leaders following Argentina's failed invasion of the Falklands.

Bignone took office in a short ceremony at the presidential palace without the support of the Navy and Air Force, which withdrew from the government last week amid bitter infighting among military leaders.

Following the failure of last-minute negotiations to draw the Navy and Air Force back into the government, political analysts said Bignone's administration held the weakest base of support in the history of military rule here as it confronts a range of grave economic and social problems.

Moments after pronouncing a simple "Yes, I swear," in answer to an oath read by Army Commander in Chief Cristino Nicolaides, the lanky, white-haired Bignone told reporters that the military divisions "logically are a matter of concern" and that healing them would be his first priority.

He also confirmed the lifting of a six-year-old ban on political activities and said he intended to work closely with civilian, political and labor leaders in formulating his government's policies. Political activity had been resuming sporadically before the April 2 invasion of the Falkland Islands.

Bignone also has pledged to pursue Argentina's campaign to claim the Falkland Islands from Britain but is expected to signal London that Argentina does not intend to continue military action, thus allowing the release of 1,000 Argentine officers still held on the Falklands.

Leaders of the populist Peronist movement, which has grown increasingly cool toward the Army government, ignored the swearing-in ceremonies and sponsored a series of events commemorating the eighth anniversary of the death of three-time president Juan D. Peron.

Peronist party Vice President Deolindo Bittel read a message at the cemetery where Peron is buried saying the armed forces had "infringed on institutions, destroyed the economy . . . exalted dictatorship and desecrated democracy." He vowed that Argentina's largest political force would not accept any resolution of the Falkland Islands conflict not approved by a democratically elected congress.

While thousands gathered at Peron's gravesite, there appeared to be little public interest in the inauguration of Bignone, 62, the seventh military man to serve as president since the March 1976 overthrow of the last Peronist government. Dozens of police manned barricades this morning around the Plaza de Mayo in front of the presidential palace to prevent crowds from assembling, but there was no attempt either to cheer or protest the new president.

Political sources said the major parties and unions were expected to begin attacking Bignone's government in the coming weeks because of the lack of an accord with the Army on policy or on the new Cabinet, which contains no representatives of principal parties. Unrest also continues in both the Navy and Air Force as well as among Army colonels and one-star generals, the sources said.

Much of the opposition to Bignone and the powers backing him--Army Commander Nicolaides and the 10 ranking division generals--is built around the perception that Bignone intends to continue with such policies as cordial relations with the United States and a free-market, anti-inflationary economic policy.

Bignone alarmed civilian leaders this week by naming as his economics minister Jose Dagnino Pastore, a minister 12 years ago under a conservative military government who is perceived as being a moderate proponent of the economic policies followed by the military in the past. These measures are fiercely opposed by Peronist politicians and labor leaders, who favor government-directed efforts to stimulate Argentine industry and raise workers' wages.

Reports appearing here today said Pastore's main priority in the coming months would be to boost exports while attempting to refinance the $8 billion due in the next six months on the massive foreign debt, estimated at $34-36 billion.

Bignone's new foreign minister, Juan Aguirre Lanari, who was recalled from his job as ambassador to Venezuela, pledged yesterday to make stronger relations with Latin America "my first priority" and said Argentina's continuing "diplomatic battle" to claim the Falkland Islands would continue "in all of its aspects."

Although the commanders of the three armed services met earlier this week in an attempt to repair divisions that led to the collapse of the joint ruling junta, the effort failed and only increased tension among military chiefs, according to political and military sources.

The ruling Army generals differ principally with their counterparts in the Air Force, who have maintained that far more sweeping changes must be made to recover from the Falklands conflict, save the staggering economy and return the country to a democratic government that will be strong enough to survive.