MEXICO CITY (Reuters) - The president of Mexico's ruling Institutional Revolutionary Party, or PRI, announced his resignation on Monday, two weeks after the party of President Enrique Pena Nieto suffered a humiliating defeat in regional elections.

Manlio Fabio Beltrones, a former governor and veteran federal lawmaker, announced his decision to step down after the PRI won only five of the dozen governors races up for grabs in the June 5 vote. It previously held nine of them.

The results were far worse for the centrist party than most polls had forecast.

The PRI's losses included two oil-rich strongholds in the Gulf of Mexico, Veracruz and neighboring Tamaulipas, both of which have been plagued by gang violence for years, as well as Quintana Roo, home to Mexico's top tourist destination Cancun. All three states have been run by the PRI for over eight decades.

"It's time for a necessary pause," Beltrones told a news conference at the PRI's Mexico City headquarters.

"This is a responsible decision that opens space for an internal debate and allows our party to freely decide the best path forward," he said.

Beltrones, 63, is still seen as a possible presidential contender for the PRI in 2018. Pena Nieto is not allowed by law to seek a second six-year term.

In a poll published Monday by daily newspaper El Financiero, Beltrones was tied for second with 10 percent among PRI voters, trailing Interior Minister Miguel Angel Osorio Chong, who had 32 percent support.

(Reporting by David Alire Garcia; Editing by Mary Milliken)