(Adds judge rejecting request to delay execution)

By Julia Symmes Cobb

MEXICO CITY, Jan 21 (Reuters) - The father of a Mexican due to be executed in Texas on Wednesday begged for his son to be spared, insisting he is innocent and saying his family is praying for a re-trial. The Mexican government has said the death sentence in this case violates international law.

Hector Tamayo told local radio on Tuesday that he will visit his son, Edgar Tamayo, on Wednesday, and that he has faith God will halt the execution.

On Tuesday, a U.S. federal judge in Austin, Texas, rejected a request to delay it.

"The court concluded that the (parole) board's procedures provided Tamayo adequate due process in conformance with current Supreme Court precedent," U.S. District Judge Lee Yeakel wrote in a three-page opinion.

Edgar Tamayo, 46, was convicted of killing a Houston police officer in 1994 while he was in the United States illegally.

"It is unjust what they want to do, knowing full well that he is not the guilty one, that he didn't kill the cop," said Hector Tamayo, speaking from Texas. "We have faith in God that the execution will be stopped and there will be a new trial.

"He tells us, his mom and I, that we must be strong, that only God has the last word."

The Mexican government said on Sunday the execution would violate a 2004 ruling by the United Nations' International Court of Justice. It has demanded that the death sentences of 51 Mexican inmates, including Tamayo, be re-examined.

The Court ruled the prisoners were not made aware of the right to consular assistance at the time of their arrests.

Two of the 51 inmates have already been executed.

The Mexican Foreign Ministry said it attempted to stay the execution through various channels, including petitions in Texas courts and from high-ranking Mexican officials.

The case has drawn attention from across the world. Tamayo said his family had received letters of support from at least 67 countries.

In September, U.S. Secretary of State John Kerry wrote a letter to Texas Governor Rick Perry urging him to reconsider Tamayo's execution because it could make it more difficult for the United States to help Americans in legal trouble abroad.

Texas has argued that it is not bound by the International Court of Justice ruling. (Reporting by Julia Symmes Cobb in Mexico City and Jim Forsyth in San Antonio; Editing by Simon Gardner, Andre Grenon, Toni Reinhold)

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