Mexicans have started casting their votes after a campaign marred by assassinations and scandals that displayed drug cartels' power.

The party that ruled Mexico for 71 years hoped to capitalise on frustrations over the bloodshed and gain momentum in its bid to regain the presidency in two years.

The Institutional Revolutionary Party (PRI), which held on to power through a system of largesse and corruption that many considered a quasi-dictatorship, has recovered popularity amid frustration with Mexico's surging drug gang violence.

The elections for governors, mayors and local posts are the biggest political challenge yet for Felipe Calderon, the Mexican president, who is deploying troops and federal police to wrest back territory from drug traffickers.

Calderon urged Mexicans to vote and show they will not be intimidated. But many say they are afraid. Scores of election workers have quit in the northern state of Tamaulipas, because they were afraid to show up at polling stations.

The PRI held up the assassination of its gubernatorial candidate in Tamaulipas as evidence Calderon has failed to bring security despite the presence of thousands of troops in drug-trafficking hot-spots.

The PRI nominated Torre's brother Egidio to run in his place. But many voters felt the cartels had snatched their right to choose - a new low in a state where henchmen extort businesses and people avoid highways where caravans of armed men travel openly.