Ricardo Najera, a spokesman for the attorney-general's office, said Sanchez is accused of aiding drug trafficking, engaging in organized crime and making transactions with illicitly obtained funds.

'Unprecedented' case

The arrest makes Sanchez, who took a leave of absence from his mayor post to run for governor of Quintana Roo state, one of the most high-profile officials implicated in Mexico's drug trade.

He is suspected of offering information and protection to the Zetas drug gang and the Beltran Leyva cartel, which are active in Quintana Roo.

Although Mexico's upcoming elections have been marred by violence and allegations of drug cartel involvement, officials said they could not remember another case in which a candidate for governor had been arrested on drug charges.

"This takes us all by surprise, it is unprecedented,'' The Associated Press news agency cited Felix Gonzalez Cantu, the governor of Qunitana Roo, as saying.

Election threats

A website run by the Democratic Revolution party, which Sanchez is a member of, published an article following the arrest, saying his detention was an attempt to damage popular support for the opposition party ahead of the July 4 poll.

The site also quoted Sanchez as saying he had been threatened.

"Resign from the race, or we are going to put you in jail or kill you,'' Sanchez said, describing one of the threats against him.

Sanchez, a populist who pledged to bring services to the impoverished majority of residents who live on the outskirts of the resort city, has challenged the long-ruling Institutional Revolutionary Party.