Julio Cesar Turbay, a veteran leader of the ruling Liberal Party, emerged as the probable winner yesterday in presidential elections officially too close to call with 95 percent of the ballots counted.

Colombia - which with neighboring Venezuela shares the distinction among South America's major republics of maintaining democracy - thus opted for a definitive break in its peculiar system under which the Liberal and Conservative parties have alternated in the presidency for the last 20 years.

Although the final count will not be known until next Sunday, the official results with 95 percent of Sunday's vote reported shows Turbay with, 2,226,163 votes to 2,137,987 for Conservative candidate Belisario Betancur.

Seven lesser candidates shared a small portion of the vote in this nation of 25 million. Almost two-thirds of the eligible electorate abstained, reflecting a malaise that is seen as a major threat to survival of the democratic tradition here. Many voters apparently preferred to stay home and listen to the broadcast of the World Cup soccer series in Argentina.

The official count see-sawed several times, with Betancur proclaiming himself victor at one point and both sides alleging instances of fraud that could complicate official designation of a winner.

Both Turbay, 62, and Betancur, 56, are slightly to the left within their parties and not far apart ideologically. Turbay has called for continuity in the policies of President Alfonso Lopez Michelsen, who was constitutionally forbidden to run again.

Four years ago, Lopez Michelsen won by a landslide. But he gradually lost the popular mandate, following severe inflation, heavy unemployment, widespread corruption and low wages of the labor force.

Turbay reaped the backlash of popular discontent and was further affected by divisions within his party. Former president Carlos Lleras Restrepo, defeated by Turbay in a primary, withheld his support on Sunday.

Despite this, Turbay appears to have put a definitive end to the system of alternating the presidency that the two traditional parties agreed to as a means of ending a virtual civil war that saw perhaps 200,000 killed in the 1950s.

The pact has been dissolving gradually since 1970, when former dictator Gustavo Rojas Pinilla challenged it by running for the presidency and coming within 60,000 votes of winning. He was in the lead when troops interrupted the counting and the result is still widely considered to have been tainted.

This time, tensions were high in the last weeks of campaign, with bombings of party headquarters, strikes and student demonstrations. But the voting was carried out in a carnival atmosphere and yesterday the capital was calm.