CARACAS (Reuters) - The Secretary General of the Organization of American States (OAS) on Tuesday severely criticized the head of Venezuela’s electoral board in a harshly-worded letter saying authorities were failing to ensure fair elections in December.

Organization of American States (OAS) Secretary General Luis Almagro gives a speech during a plenary session of Mexico's Senate in Mexico City, September 8, 2015. REUTERS/Edgard Garrido - RTX1RP6P

The OAS' Luis Almagro wrote a 19-page letter to Tibisay Lucena, who heads Venezuela's National Electoral Council (CNE), urging her to level the playing field between the Socialist Party and opposition. (Letter in full: goo.gl/rbFuS0)

“There are reasons to believe that the conditions in which people will vote ... aren’t right now as transparent and just as the (electoral council) ought to guarantee,” wrote Almagro.

He was responding to a letter from Lucena which was not made public. The CNE did not respond to a request for comment.

Government critics say authorities are trying to tilt the Dec. 6 legislative elections toward the Socialist Party, which could lose control of parliament as President Nicolas Maduro struggles under a collapsing state-led economic system.

Almagro said that Maduro’s governing Socialist Party has unfair electoral advantages in its use of public resources in the campaign, access to the press, confusion in voting cards and the disqualification of some opposition political figures.

In September, Leopoldo Lopez was sentenced to nearly 14 years in jail on charges of inciting anti-government protests last year that spiraled into violence killing more than 40 people.

“It’s worrying that ... the difficulties only impact the opposition parties,” Almagro wrote. “You (Lucena) are in charge of electoral justice. You are the guarantor.”

The OAS has said it would be happy to observe Venezuela’s elections, though the CNE has rejected the offer.

Only the Union of South American Nations (UNASUR) is to send a mission to Venezuela to observe the vote.

Almagro alleged that this denial of international observers is “political positioning.”

Nationwide polls show voters are more than twice as likely to back opposition candidates than those of the Socialist Party, according to local media reports of polls conducted in August and September.

But no pollsters have published surveys in each electoral circuit, which experts say is the only way to accurately measure public opinion for this type of vote.

Maduro, who describes the OAS as a puppet of Washington, has denied accusations of electoral advantage. He accuses the OAS of “meddling” in Venezuela’s affairs.

Until early this year, Almagro was foreign minister in Uruguay, one of Venezuela’s closest trading partners.