After a surprisingly strong showing for Chilean conservatives in presidential elections tonight, right-of-center economist Joaquin Lavin forced a runoff race next month with Socialist Ricardo Lagos, a dissident during the military dictatorship of Augusto Pinochet.

With 99.33 percent of the ballots counted, the two were virtually tied. Lagos, also an economist, had 47.96 percent of the vote, compared to 47.52 percent for Lavin, a former supporter of Pinochet who has now largely disavowed the ex-dictator. That's the strongest showing for a right-wing presidential candidate in decades.

Four other candidates had small percentages. But since no candidate won a majority of the vote, the winner will be determined by a final round of voting Jan. 16. Recent opinion polls had predicted that a second round would be required, but Lagos was widely expected to win the first round by at least 3 percentage points.

Lagos, 61, is trying to become Chile's first Socialist president since Salvador Allende was overthrown by Pinochet in 1973. Lavin, 46, an economist educated at the University of Chicago who has successfully portrayed himself as nontraditional centrist, is trying to break down Chile's political barriers.

"Tonight were are witnessing the birth of a new kind of political system, a country without a right or a left, no fights between the rich and poor, a country at peace," Lavin said.

In a country where the right wing was long equated with the oppression of Pinochet, the strength of Lavin's candidacy seemed to shock both Lagos's camp and the nation. But after 10 years of rule by the center-left Concertacion coalition that Lagos's Socialists now lead, voters responded to Lavin's populism, nationalism and promise to close the gap between the rich and poor and to fight poverty.

Critics have said that those promises will be difficult to deliver, considering that his base is rooted in the wealthy right-wing elite. Many of Lavin's core backers also continue to embrace the legacy of Pinochet's brutal regime.

Lagos had put many of the same issues at the center of his campaign, but Lavin was better able to deliver with youthful, energetic, grass-roots campaigning backed by an estimated $40 million-war chest--several times that of Lagos's. Lavin's message appeared to strike a chord, especially with young voters and conservative members of the governing coalition.

Lagos made a public appearance tonight well before the vote counting was finished to appeal to Chileans--especially the members of his governing coalition who had clearly broken ranks to vote for Lavin. "We need to unite all Chileans who want equality and . . . who want to leave behind a Chile controlled by a [elite] minority of the people," he said.

But Lagos's lieutenants acknowledged that the campaign needed a jump-start to compete with Lavin's slick, well-scripted and aggressive campaign. "We need to strengthen the [details of the campaign] and it's been late, and we're going to do it now," said Sen. Sergio Bitar, a member of the left-wing Party for Democracy and a close Lagos ally.

Next month's winner will replace President Eduardo Frei, the outgoing president from the centrist Christian Democrats.

The race marked the first time a presidential election has gone to a second round since the end of Pinochet's dictatorship in 1990, when the current electoral system was put in place. The race has become the subject of international focus as Pinochet sits under house arrest in London fighting extradition to Spain, where a magistrate wants him tried for crimes committed during his regime. More than 3,000 dissidents disappeared or were killed during Pinochet's 17-year rule.

But the campaign was defined by the candidates' views on domestic issues such as unemployment and education. Analysts suggested tonight that Lagos, who only a few months ago enjoyed a large lead in opinion polls, perhaps focused too much on lofty ideals such as human rights and the need to deepen democracy, which is still limited by a constitution drafted during Pinochet's tenure.

Lavin tried to shift what he called "political issues" to the periphery of the national agenda, arguing that the government should focus more on improving efficiency and fighting unemployment.

Although Lavin clearly has the momentum going into the second round, some analysts said Lagos may be helped if he takes at least a portion of the 3.19 percent of the vote won by Communist candidate Gladys Marin, who finished a distant third.

Marin has urged her Communists to cast blank ballots to protest Lagos's embrace of the free market and his rejection of negotiations with the Communist Party. Analysts said such talk would cost Lagos support of members of the centrist Christian Democrats, the largest party in his coalition.

"I see Lagos as getting stronger in the second round," said Enrique Correa, a Santiago-based political analyst. "Lavin has run a smart campaign, but I think Lagos has been able to generate more faith in the people."

Lagos and Lavin have both pledged to forge ahead with Chile's free market economy, which has become a model in the developing world but this year, along with much of the rest of Latin America, sunk into its worse recession in a decade. Both candidates have also focused on the need to close the vast chasm between the rich and the poor, which has widened in this country of 12 million even as Chile posted record economic growth this decade. However, they have disagreed on how best to accomplish those goals.

Though Lagos is a Socialist, his policies bear almost no resemblance to those of Allende, who advocated land seizures and the nationalization of private industry. Lagos has stressed the need to reinforce the state's role in society, arguing that the free market has not succeeded in creating a more equitable country.

Lagos, who earned a doctorate in economics from Duke University, made history in the late 1980s as one of the only dissidents bold enough to challenge Pinochet publicly. During a national plebiscite on whether Pinochet's rule would last another six years, Lagos denounced the dictator on national television. Soon after, Chilean voters overwhelmingly rejected Pinochet's rule, and the military leader stepped down in 1990.

CAPTION: Chilean Socialist presidential candidate Ricardo Lagos greets supporters. He is headed into a runoff election next month with conservative Joaquin Lavin.

CAPTION: Conservative Chilean presidential candidate Joaquin Lavin waves to voters he attracted with a call to improve government efficiency and provide jobs.