The opposition's strong showing in presidential elections marks a watershed in Mexican politics - a transition from 59 years of a one-party system and the awakening of a long passive electorate, political analysts say.

Early returns indicated that Carlos Salinas de Gortari, candidate of the governing Institutional Revolutionary Party, would finish with no more than 52 percent of the vote, party officials say. That is far below the 70 percent the party had considered the minimum.The PRI has not lost a presidential, gubernatorial or senate race since it was founded in 1929. Partial returns have shown Salinas leading with 47.8 percent of the vote.

PRI officials appeared willing to concede that Cuauhtemoc Cardenas of the leftist National Democratic Front and Manuel J. Clouthier of the conservative National Action Party would finish better than original polls indicated.

PRI sources, who spoke on condition of anonymity, conceded Salinas had likely lost in Michoacan, the Federal District, and adjoining Morelos state and probably in the state of Mexico. The Mexico City metropolitan area accounts for about 20 percent of the 38 million registered voters.

Cardenas' and Clouthier's parties said Salinas lost, and they claimed widespread fraud.

The voter turnout and final results were not expected until Sunday, but partial results were leaked in an attempt to defuse allegations of manipulation of the vote.

Salinas insisted on a small but credible margin of victory rather than the frequently fraud-tainted results the party is known for. But the almost certain loss of congressional seats and the opposition entry into the Senate for the first time is hard for the party's old guard.

"Psychologically, it means a big blow," Adolfo Aguilar, a Mexican economist with the Carnegie Endowment for Peace, said. "Politically, just imagine the slice of people they have to cut. This chunk of votes means many PRI candidates. It's going to be disastrous for the PRI."

Political analyst Jorge Castaneda said Cardenas did much better than expected, particularly in urban areas.