The candidate of an opposition party associated with wartime atrocities scored highest in Guatemala's first presidential election since a protracted civil conflict ended here in 1996. But results of Sunday's first round of voting showed today that the candidate, Alfonso Portillo, will be forced to seek confirmation of his victory in a runoff next month.

Portillo, a 48-year-old lawyer, economist and law-and-order candidate of the Guatemalan Republican Front (FRG), failed to capture the majority needed to avoid the second round. With 97.5 percent of the votes tallied, Portillo had received almost 47.8 percent of the total, compared with 30.3 percent for businessman and former Guatemala City mayor Oscar Berger, 53, the candidate of the ruling National Advancement Party (PAN), election officials reported.

That meant Portillo and Berger appeared headed for a runoff on Dec. 26. Alvaro Colom, 48, of the New Nation Alliance, a coalition that includes some former leftist guerrillas, drew only 12.2 percent of the vote.

The Supreme Electoral Tribunal said 53.9 percent of the country's 4.5 million voters went to the polls, one of the highest participation rates ever in Guatemala. Analysts said the turnout reflected stepped up registration efforts and discontent among Guatemalans three years after peace agreements ended Central America's longest civil conflict.

An estimated 200,000 people, mostly indigenous peasants, were killed or disappeared during the 36 years of fighting.

Portillo recently admitted that he fatally shot two men--in what he said was self-defense--during a fight in Mexico 17 years ago, and then fled out of fear of being unfairly prosecuted. A judge closed the case in 1995, and Portillo deftly turned the incident into an asset as part of a populist campaign that gained the support of many Guatemalans who view the government as elite, corrupt and incapable of delivering the social and economic dividends they hoped would come with peace.

Portillo prevailed in all of Guatemala's 22 states and the capital, traditionally a PAN stronghold, despite the fact that the Republican Front's founder and secretary general, former coup leader Efrain Rios Montt, 74, has been accused of sanctioning widespread killings of guerrillas and suspected supporters by the military when he was president in the early 1980s.

"The outcome of the election was a vote against arrogance . . . the abuse of power and authoritarianism," Portillo, who is favored to win the runoff, told reporters today. President Alvaro Arzu was constitutionally barred from seeking reelection.