Luisa Maria Calderon who lost the vote immediately questioned the results [Reuters]

Mexico's former ruling party has won a key governor's race, defeating President Felipe Calderon's sister in a race seen as a key indicator of the possible outcome of next year's presidential vote.

Preliminary results from the Sunday gubernatorial election in the western state of Michoacan showed that Fausto Vallejo Figueroa of the Institutional Revolutionary Party, or PRI, won 35 per cent of the vote. Luisa Maria Calderon of took 33 per cent, according to electoral authorities.

The win for PRI is a major step toward regaining the presidency it lost in 2000 after governing Mexico for 71 years.

Silvano Aureoles, from the leftist Party of the Democratic Revolution (PRD) which has governed the state since 2002, was third with 28.9 percent.

The two parties that lost the vote immediately questioned the results and accused the PRI of aligning itself with organised crime to intimidate voters. Aureoles called for the result to be annulled.

Calderon, a candidate for her brother's conservative National Action Party, or PAN, said her team would carefully review vote tallies in parts of the state where they had received reports of armed men threatening people trying to vote.

"Allowing organised crime to manipulate elections will never lead to security," she said in an interview with the Televisa network.

Federal prosecutors said they have opened investigations into 42 alleged instances of voting irregularities, including threats purportedly used to force people to vote a certain way and people holding others' voter ID cards, which is done to prevent people from voting or to allow others to vote for them.

The PRI's Vallejo responded that his backers were also subjected to criminal threats.

"No one was exempt,'' said Vallejo, who had been mayor of the state capital, Morelia.

The PAN was seeking a symbolic victory in Calderon's home state, where he launched the assault against cartels in late 2006.

Michoacan is home to two of Mexico's most powerful drug cartels, La Familia and the Knights Templar, which claim to be following divine will and protecting the rights of the state’s residents.

The PAN mayor of the city of La Piedad was gunned down as he handed out campaign literature for Calderon and other candidates less than two weeks before Sunday's election.

The drug war has killed more than 40,000 people according to estimates, although no official figures have been released in nearly a year.

Source: Agencies