By LARRY ROHTER
 
MONTEVIDEO, Uruguay, Oct. 31 - Tabaré Vázquez, a Socialist doctor running as the candidate of an opposition coalition that includes former guerrillas, narrowly triumphed Sunday in the presidential election, bringing the left to power for the first time in this South American country.

The victory by the coalition, known as the Progressive-Encounter-Broad-FrontNew-Majority, whose largest faction consists of Tupamaro guerrillas turned politicians, strengthens a trend throughout the continent. As in the last presidential votes in Venezuela, Brazil, Ecuador and Argentina, the candidate most opposed to American-supported free-market policies has defeated backers of those policies.

Surveys of voters leaving the polls and early returns indicated that Dr. Vázquez, an oncologist and former mayor of this capital, would win about 51 percent, just above the majority needed to avoid a runoff. Even before official returns were announced, both of his main opponents had conceded and indicated their willingness to cooperate with him.

"Celebrate, Uruguayans, celebrate," Dr. Vázquez, whose five-year term is scheduled to begin March 1, told the throng by his campaign headquarters at the downtown Hotel Presidente, two hours after polls had closed. "This victory is yours!"

Tens of thousands of people, some with faces painted in the red, blue and white colors of the Front, took to the streets here, setting off firecrackers, waving banners, honking horns and pounding drums. "We did it, we finally did it!" shouted Walter Correa, a meatpacking plant worker.

The triumph caps a 33-year effort by the Front to win power by legal means. During the American-supported right-wing military dictatorship that ruled from 1973 to 1985, many Front leaders who were then Tupamaros, as well as members of some other factions, were jailed or forced into exile.

The son of a politically conscious oil refinery worker, Dr. Vázquez visited his father's grave before voting in the working-class neighborhood where he was born and reared. He began his medical and political career in the same district.

"This is a magic night," Dr. Vázquez said at a news conference, paying tribute to Front leaders who did not live to see his victory. He also urged countrymen to "unite our efforts so that all Uruguayans can live better."

For Uruguay's two traditional centrist political parties, the Colorados and Blancos, the defeat was traumatic. Together, they have alternated in the presidency for more than a century, but were outvoted even in some middle- and upper-class neighborhoods that historically have supported them.

"The Front clearly has the best of the packages being offered to us, and Tabaré Vázquez is obviously a balanced and capable leader," said Gonzalo Mendoza, 60, an architect. "This country has been in misery for the past 20 years, but now we will have a government that recognizes that social policy is important and that the economy is not the be-all and end-all of everything."