But the leftist opposition claimed 'enormous fraud' - traditional in Mexican elections - and questioned whether Mr Zedillo would be able to take office in December, because a majority of Mexicans had rejected the PRI for the first time in 65 years.
The PRI candidate declared victory at 3am yesterday with just 15 per cent of the votes in. Later, with one-third counted, he had 47 per cent, ahead of the conservative National Action Party (PAN) with 30 per cent and the leftist Democratic Revolution Party (PRD) with 15.
The PRD leader, Cuauhtemoc Cardenas, held a demonstration in Mexico City's main square and told 15,000 supporters he did not accept defeat. He accused the PRI of using the 'ants' carousel' - multiple voting by supporters.
Angry citizens barred from voting had burned ballot boxes, smashed the windows of public buildings and claimed massive fraud across the nation on Sunday.
Mr Zedillo's 47 per cent surprised many of his countrymen and foreign observers, who had expected a close three-way race. 'They've done it again. And we didn't even see their hands move,' said one left-wing South American election observer.
Mr Cardenas was widely thought to have won the 1988 presidential race when a mysterious computer failure at the federal election centre turned his lead into victory for the PRI.
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