By Will Dunham

WASHINGTON (Reuters) - Jeremiah Denton, a former U.S. senator who was held as a prisoner of war by North Vietnam for more than seven years and revealed his treatment by blinking the word "torture" in Morse code during a televised interview, died on Friday at age 89.

Denton died at a hospice facility in Virginia Beach, Virginia, from a heart ailment, his family said.

The retired Navy rear admiral was elected in 1980 as Alabama's first Republican senator in 112 years and earned a reputation as one of the Senate's most conservative members before being defeated in his 1986 re-election bid. President Ronald Reagan lauded him that year as "a national treasure."

President Barack Obama said in a statement that Denton "leaves behind a legacy of heroic service to his country."

"The valor that he and his fellow POWs displayed was deeply inspiring to our nation at the time, and it continues to inspire our brave men and women who serve today," Obama said.

Denton was most famous for spending seven years and seven months as a Vietnam War POW after his plane was shot down during a bombing mission from the aircraft carrier USS Independence in 1965. Imprisoned in brutal conditions in and around Hanoi, Denton encouraged fellow American prisoners to resist their North Vietnamese captors.

American POWs were sometimes paraded in propaganda films and in 1966, the captive Denton was interviewed for such a film - it later aired on U.S. television - apparently in the hope that he would denounce the U.S. war policy.

"Well, I don't know what is happening," he told his interviewer. "But, whatever the position of my government is, I support it fully. And whatever the position of my government is, I believe in it - yes, sir. I'm a member of that government and it is my job to support it. And I will as long as I live."

During the interview, he pretended to have light sensitivity that caused him to blink his eyes. What he was actually doing was blinking in Morse Code to spell out "t-o-r-t-u-r-e."

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Denton said later his torture increased after the interview was aired. He spent four years in solitary confinement, including two years in a cell the size of a refrigerator. He was 41 when he was captured and 48 when released.

Denton, one of the highest-ranking U.S. officers to become a POW in Vietnam, was a Navy commander at the time of his capture and was promoted to captain while imprisoned.

He was among the first U.S. POWs released by the North Vietnamese in February 1973 under the Paris Peace Accords.

'PROFOUNDLY GRATEFUL'

"We are honored to have had the opportunity to serve our country under difficult circumstances," Denton said after he stepped off the first plane to arrive at a U.S. base in the Philippines. "We are profoundly grateful to our commander in chief and to our nation for this day. God bless America."

He was later promoted to rear admiral and headed the Armed Forces Staff College in Virginia until retiring from the U.S. military in 1977. He wrote a book about his time as a prisoner, "When Hell Was in Session". It was made into a 1979 TV movie with Hal Holbrook portraying Denton.

Republican Senator John McCain of Arizona, another former Vietnam War POW who went on to leave his mark in U.S. politics, recalled Denton as "my friend and mentor."

"As a senior ranking officer in prison, Admiral Denton's leadership inspired us to persevere, and to resist our captors, in ways we never would have on our own. He endured unspeakable pain and suffering because of his steadfast adherence to our code of conduct," McCain said in a statement on Friday.

Republican Senator Jeff Sessions of Alabama on Friday called Denton "a man of grit and character that can't be manufactured."

Former Republican Senator Pete Domenici, who served with Denton in the Senate, added, "He brought an experience and credibility with him that immediately established him as the kind of American who was willing to do everything he could to help the country, putting his own interests second."

Denton, a devout Roman Catholic and the father of seven, became disenchanted after his release with what he viewed as a decline in American values including drug use, legalized abortion and pornography. He was elected to the U.S. Senate from his native Alabama, serving from 1981 until 1987.

As a senator, Denton was a strong backer of Reagan's efforts to support rebels fighting Nicaragua's leftist government. He also worked to limit government funding of abortions and to let religious groups use public school property on equal footing with other groups.

He was defeated in 1986 by Richard Shelby, a Democratic congressman who later became a Republican.

"I'm not a right-wing nut," Denton said in a 1980 magazine article. "But politics is like a pendulum. We have a conservative swing in this country, and it is time to act. Citizens have their own self-respect and want this nation to regain hers."

Denton's wife of more than 60 years, Jane, died in 2007.

(Additional reporting by Colleen Jenkins; Editing by Bill Trott, Stephen Powell and Andrew Hay)