Showing little of the exuberance or emotion that characterized the start of his previous trips to Mexico and Africa, Pope John Paul II arrived here today at the start of an historic 12-day visit to Brazil, the world's most populous Catholic country and home of an increasingly activist church.

Although some liberal church leaders had expressed fears that the pope might try to rein in their activities on behalf of landless peasants, impoverished workers and disenfranchised Indians, John Paul reassured them today when he called on Brazil to over-come "the suffering and bitterness of the present."

In the presence of Brazil's military president, Joao Figueiredo, who greeted the pontiff after the long flight from Rome, John Paul called on the Brazilian people and government "to build an exemplary social harmony, overcoming disequilibriums and inequalities.

"In justice and concord, with lucidity and courage, without shocks or ruptures, this certainly will be in eminent service to international peace and, therefore, to mankind," the pope said in his opening remarks, shortly after he arrived in Brasilia at the end of an arduous 12-hour flight.

During his stay in Brazil, the pope is expected to try to define more precisely the line he has attempted to draw between church support for "social justice," the "option for the poor," and his own prohibition on the church or priests getting directly involved in partisan politics, political parties or political disputes.

Despite impressive economic and social gains over the past 15 years, Brazil is a country where the distribution of income is badly skewed in favor of the rich, and where 30 million people are believed to the be living in "absolute poverty." The church has become active in attempting to ameliorate the "inequalities" to which the pope referred by supporting unions and other groups against the government and entrenched business interests.

John Paul's visit has been awaited with much anticipation by all sectors of society here. His words concerning the church's role in supporting social causes could have a significant impact on the country's political and economic future.

The pontiff was greeted at the airport today, by Brazil's most important government, military and church leaders. He then boarded an open car for an hour-long drive into the heart of Brazil's futuristic capital where he celebrated the first of 12 masses at which he is scheduled to officiate during his almost two weeks in Brazil.

Thousands of Brazilians lined the route, cheering and shouting as the pope waved from his car. Many of the children were dressed in their best clothes for what was likely a once-in-a-lifetime opportunity to see John Paul, the first pope to visit Brazil.

Large crowds gathered on the ministerial esplandade at the center of Brasilia where a huge outdoor altar had been erected for the pope to celebrate mass. Brazil's population of 120 million is about 95 percent Catholic. Many Brazilians also participate in macumba and other voodoo rites brought to this country by African slaves imported by the Portuguese in the 16th, 17th and 18th centuries.

Although the pope said in his opening remarks that "I find myself here on a mission that is clearly pastoral and religious," he was greeted today as the Vatican's head of state. Most of his time was spent carrying out the political, diplomatic and ceremonial chores of a visiting king or president rather than the pastoral duties that will occupy most of his remaining time in Brazil.

In the late afternoon, he was scheduled to meet alone with Figueiredo for about 45 minutes, possibly to discuss the deep antagonisms that currently exist between the military government here and the Brazilian church, which is considered the most liberal in Latin America.

Once John Paul leaves Brasilia Tuesday, the emphasis of the visit will change as he goes among the people to demonstrate his interest and concern for the social and religious issues that affect Brazil.