UPDATE 2-Venezuela arrests head of U.S. refiner Citgo in graft sweep

(Adds details on arrests, Citgo deal, Saab quote)

CARACAS, Nov 21 (Reuters) - Venezuelan authorities on Tuesday arrested the acting president of its U.S.-based refiner Citgo and five of the subsidiary's top executives as part of a spiraling corruption purge in the OPEC country's oil industry.

Military intelligence agents detained Jose Pereira during an event at state oil company PDVSA's headquarters in Caracas, two sources told Reuters, in the latest of dozens of high-level arrests in the last few months.

State Prosecutor Tarek Saab has declared a "crusade" against "organized crime" within PDVSA, and has now arrested around 50 oil managers since taking office in August.

In the case of Texas-based Citgo, Saab told a news conference that his office had uncovered a roughly $4 billion planned deal with foreign firms, offering the refiner as guarantee in a detrimental deal for Venezuela.

The deal was with U.S. investment fund Apollo Global Management LLC and Dubai-based Frontier Management Group LTD, and also included a Swiss-based intermediary, Mangore Sarl, according to Saab. He added there was a "presumed" link between Mangore Sarl and the Citgo executives.

"This board of directors put Citgo in danger. That's corruption, corruption of the most rotten nature," Saab said.

It was not immediately possible to contact Pereira or the other arrested executives. Citgo did not immediately respond to a request for comment.

Frontier Management Group LTD did not respond to a phone call. Apollo Global Management LLC did not respond to a request for comment by email. A call to a phone number for Mangore Sarl listed on a Swiss web site focused on commercial entities was not answered.

President Nicolas Maduro's government and PDVSA, which is formally known as Petroleos de Venezuela SA, have repeatedly vowed to take steps to combat corruption.

Opposition leaders say PDVSA has been crippled by malfeasance under 18 years of socialist rule. They attribute the arrests to in-fighting among rival government factions rather than a genuine desire to root out corruption.

Sources close to PDVSA also said the arrests owe more to turf wars for control of the company, which is the financial motor of Venezuela's ailing economy.

Saab, who replaced a dissident former state prosecutor who broke with the government and fled abroad this year, disputes that, saying political opponents are seeking to discredit his push for justice.

During his presentation on Tuesday he blasted "those writers who, in an indignant manner, are trying to create a false narrative." (Reporting by Alexandra Ulmer; editing by Andrew Cawthorne and Marguerita Choy)