HUNTSVILLE, Texas, Aug 14 (Reuters) - Texas executed a Mexican citizen on Wednesday for the 1988 murder of an undercover Dallas police officer despite pleas for his life from Mexican President Vicente Fox.

Javier Suarez Medina, 33, was put to death by lethal injection in the state prison in Huntsville, 75 miles (121 km) north of Houston, after the U.S. Supreme Court rejected a last-ditch legal appeal from Mexico, and Texas Gov. Rick Perry refused Fox's request for a reprieve.

Following the execution, Fox canceled a three-day trip to Texas that he had been scheduled to begin on Aug. 26. Rodolfo Elizondo, Fox's spokesman, said the cancellation was meant as "an unequivocal sign of our rejection of the execution."

A calm-looking Suarez, strapped to a gurney in the Texas death chamber, asked the family of his victim, Dallas narcotics officer Lawrence Cadena, to forgive him.

"I would like to apologize to the family members of the Cadena family for whatever hurt and suffering I have caused you," he said. "I thought about your loved one very much. He will be waiting for me in heaven. I will be able to talk to him and ask him for forgiveness personally."

Suarez then began to sing "Amazing Grace" before he was overcome by the lethal drugs.

Suarez was the fifth Mexican executed in Texas in the past 20 years in what is becoming an increasingly sensitive issue for U.S.-Mexican relations, as shown by Fox's intervention. He pleaded with Perry and friend U.S. President George W. Bush to stop the execution, calling it his government's "highest priority." But it was to no avail.

Afterward, a somber Eduardo Ibarrola, Mexican consul general in Houston, told reporters: "Mexico rejects the application of the death penalty and believes that it resolves nothing."

Suarez was condemned for fatally shooting Cadena, 43, during a buy-and-bust drug sting in a Dallas parking lot on Dec. 13, 1988. Suarez, who was 19 at the time of the crime, admitted that he killed Cadena, but said he did not know Cadena was a police officer.

'GOD WILL NOT LET THIS HAPPEN'

A handful of anti-death penalty protesters shouted outside the prison as Suarez was receiving the lethal injection.

"Hello, Texas, you criminal state. One day you will pay because God will not let this happen," said one protester through a megaphone.

Mexico sought a stay of execution from the Supreme Court on the grounds that police violated Suarez's rights because he was not put in contact with the Mexican consulate after his arrest as required under the Vienna Convention international treaty.

The Supreme Court rejected the appeal without comment shortly before Suarez was executed. He was pronounced dead at 6:23 p.m. (2323 GMT).

Suarez was born in Mexico, but had lived in Texas since age 3. Dallas police said they did not put him in contact with Mexican officials because they did not know he was a Mexican.

Prosecutors say Suarez would have been condemned to die under any circumstances because he gunned down a police officer while making a cocaine delivery -- the double felony that Texas law requires to become a death penalty case.

Mexico's appeal also said that Suarez suffered cruel and unusual punishment because he was scheduled for execution 14 times, only to be saved by last-minute legal maneuvers every time until the last one.

"I've never known a case in which the person has had that many execution dates," said Suarez lawyer Sandra Babcock, a veteran of numerous death penalty cases.

Twelve Latin American nations and two in Europe -- Spain and Poland -- filed a legal brief in support of the Mexican appeal, she said. U.N. High Commissioner for Human Rights Mary Robinson joined the effort on Tuesday, urging in a letter to Secretary of State Colin Powell that the matter be reviewed.

Javier Corral, a Mexican senator from Fox's conservative National Action Party (PAN), said in Mexico City: "We dispute from the start and as a matter of principal that problems of criminality can be solved by committing another crime."

Suarez became religious in prison and told reporters recently that he would prefer to die than continue to live in the intense isolation of Texas death row near Livingston, about 70 miles (113 km) north of Houston.

Death row prisoners are allowed out of their cramped cells for only one hour per day, and always alone.

Prison officials said Suarez did not request a final meal.

Suarez was the 21st person executed this year in Texas and the 277th since the nation's leading death penalty state resumed capital punishment in 1982. Nampa-Reuters 0511 150802 WEB story ENDS (NAMPA 150515)