A new, populist president has begun to shake up this turbulent country's normally staid government with a symbol-laden campaign to "open" Colombian democracy and to distance foreign policy from its once-tight alignment with the United States.

Belisario Betancur, 59, a veteran of Colombia's tradition-bound Conservative Party, has surprised both diplomats and the political establishment by steering his three-month-old presidency away from the detached style of a generation of Colombian governments.

Instead, the symbols of populism and aggressive change now abound. Mercedes-Benz limousines have been banished from the presidential staff pool, and patronage appointments have been reversed on ethical grounds. Guerrilla leaders have visited the presidential palace and the Congress to talk of a newly legislated amnesty, and commissions are meeting to study economic and political reforms.

Betancur signed the amnesty law Friday, United Press International reported, declaring it a "mandate for peace."

At the same time, backed by lingering public antagonism to U.S. support for Britain in the Falkland Islands conflict, Betancur's government has conspicuously shifted the tone of a foreign policy that had made Colombia one of Washington's closest regional allies.

Even as President Reagan prepares to visit Colombia on a Latin American tour that is to start Nov. 30, officials here are planning an active role for this country in the Nonaligned Movement amid occasionally sharp anti-U.S. rhetoric. "It is known that we are the back patio for the United States," Betancur was quoted as saying recently. He added: "They don't know how to help, even when they want to help."

The new president's aggressive plans and bold style have won him popularity in Colombia, where in recent decades social disorder has been matched by increasing political stasis and public apathy.

"It's been a long time in Colombia since there was such good will toward a government," said Alvaro Gomez, a Conservative Party leader. "The government has raised very high hopes, to the point where it could be a problem. Everyone now expects more than could possibly be done."

Government officials concede that Betancur's elaborate attempts to project a public image of dynamism could backfire, but they argue that Colombia's 27 million people need a shake-up to restore confidence in democracy. Governments long on patronage and elite party establishments have been stained with corruption and have made little progress in developing the nation's vast and scarcely populated outlying territory or reducing economic inequalities.

Resultant apathy of the electorate has been reflected in descending voter turnouts and violence and organized crime in the countryside -- including large-scale trafficking of narcotics to the United States and Army battles with five different armed insurgent movements some of which at times have been backed with aid from Cuba.

From 1958 to 1974, the traditional Conservative and Liberal parties alternated in the presidency under a pact that ended a decade of virtual civil war. Betancur is the first Conservative president since expiration of the pact. He was elected when the Liberal Party split. So far, he has offered little more than style in attempting to reverse the government decline. But some veteran politicians here say that the president's flair for populist symbolism, such as opening the stately presidential palace to the public on weekends, is a major change.

"Politics was run here by a very cultured class, with a mastery of theory and a truly patrician style," said Alfonso Lopez Michelsen, a Liberal ex-president and one of the two candidates Betancur defeated in the June election. "But meanwhile Colombia was changing, and the politicians were left behind. It was becoming a much more informal country socially -- women started to dress in blue jeans and the influence of the Catholic Church fell off."

"Betancur has caught up with the times," Lopez added. "He finds out what people want, and then he says what they want to hear, in the language and style of the television drama. He has taken politics from the academic to the ranch house and the tango."

The shift in tone that so far seems most to disturb many of Betancur's opponents is what government officials call the new "independence" of foreign policy. Officials quickly made clear their perception that the previous Liberal government of Julio Cesar Turbay Ayala had allied itself too closely with the United States. Turbay strongly backed U.S. policy in Central America and sent Colombian troops to the Middle East for the Sinai peace-keeping force.

Colombia was also one of the few Latin American nations to vote with the United States and against Argentina in the Organization of American States during the Falklands conflict with Britain this year--a position Betancur has since renounced.

It was in fact the U.S. position in the Falklands war that provided the public impetus for a change in policy, government officials now say. "For the public, the last push against the United States was the Malvinas war," said Foreign Minister Rodrigo Lloreda Caicedo. "Paradoxically, the sentiment here was stronger against the United States than Britain."

Lloreda said in an interview that Colombia still supported U.S. programs for the Caribbean basin and Central America, and he noted that the new government had recently backed Washington in the United Nations on excluding the issue of Puerto Rico's status from that forum. But, he said, "we have seen a radicalization of the positions of the two great powers, and our philosophy is that we do not need to be aligned with either of the two blocs."

The move away from the Reagan administration following the Falklands conflict parallels that of neighboring Venezuela, another one-time U.S. ally on aid to El Salvador.

"The United States has not had a real policy in Latin America for a long time," said Rodrigo Escobar Navia, Betancur's minister of government. "The United States is acting here as a world power, in the context of emphasizing East versus West, before completing its role as a regional power."

Colombian officials say they hope Reagan's trip to Colombia, Brazil and Costa Rica will lead to a reorganization of U.S. policy and stepped-up interest in the region. Meanwhile, they say, the new Colombian government remains more preoccupied with its ambitious domestic programs than foreign policy.

Betancur has backed the broad new amnesty offer to members of Colombia's guerrilla movements and has won the endorsement of some of the leftist insurgent leaders by offering negotiations and studies of substantial political and electoral reforms.

The president also has sought to stress fiscal austerity and labor-management agreements to halve inflation from a current level of more than 20 percent. Betancur recently launched the drive in a speech in which he formally canceled Colombia's costly commitment to host the World Cup soccer championships in 1986. This year's presentation of the Nobel Prize for literature to Colombian novelist Gabriel Garcia Marquez would more than compensate for any loss in international prestige on the soccer circuit, said Betancur.

So popular is the portly, silver-haired president that the blow to a religiously followed sport raised little protest, and the politically active leftist Garcia Marquez told reporters that he felt free to return to Colombia from exile in Mexico because of the change in government.