Robert C. Ode of Falls Church -- the oldest of the American hostages held at the U.S. Embassy in Tehran -- has issued an open appeal to the Carter administration to "take prompt action to free us from this terrible situation."

In a letter to The Washington Post that was received yesterday, the 64-year-old Ode -- a retired career Foreign Service officer who had agreed to help out for 45 days in the consular section of the U.S. Embassy in Tehran -- described his confinement in stark terms.

"We are being kept in semidarkened rooms; our hands are tied day and night; bright lights are kept burning all night and because of the constant noise it is almost impossible to sleep," Ode wrote.

State Department officials, who said Ode's letter appeared to be authentic, said a new batch of at least eight to 10 letters from American hostages had been received by relatives in the United States during the last two days.

"We don't know why they are allowing the letters out," said State Department spokesman Merton Bland.

U.S. officials said they presumed that the letters -- dated as having been written between Dec. 14 and 27 -- had been mailed with the approval of the Iranian Militants who seized the U.S. Embassy Nov. 4. They said it was too early, however, to assess the significance of the arrival of the letters at this time.

But State Department officials last night described Ode's letter to The Post as the frankest they had yet seen from any hostage. According to reports from the 13 American hostages released by the Iranian militants in late November, Ode was among the most outspoken of the captives at the embassy.

In his letter to The Post, Ode said: "In 53 days I have been given only three brief exercise periods in the fresh air and only four tasteless and unripe oranges, two hard boiled eggs, one small bottle of fresh milk and a few pressed dates to supplement and otherwise monotonous and too starchy diet."

He complained that mail was being withheld from the hostages, that no U.S. government representatives had been allowed to visit them and that their captors had not permitted them to receive any news or indication of what steps the United States was taking to help them.

"I can only ask that with your power of the press that you do everything possible to bring pressure on the responsible leaders in our government to take prompt action to free us from this terrible situation," Ode said in his letter to The Post. "Anything you can do to help us will be greatly appreciated."

Ode's wife, Rita, and his sister, Marjorie Keon, of St. Louis, Mich., also received letters from him yesterday. Officials said Ode also mailed letters to the White House and members of Congress.

"I wept when I read it," said Keon, his sister. "I think the psychological damage is going to be so great that some of [the hostages] will probably never recover."

Ode's wife declined to comment, saying, "I don't want to break my silence. Everyone would be after me if I did, and there is nothing that I could say that would make any difference."

Ode, as he noted in his letter, retired from a long State Department career at age 60, but has been > called back several times since for special assignments. He agreed to go to Tehran Oct. 3 -- one month before the embassy takeover -- on what was to be only a temporary consular job. In his letter, he said he was due to return here last Nov. 15.

"Bob is a very good diplomat. He's just cut out for that kind of thing. He loves people. He loves his work," his sister said in an interview a few days ago, before his letters were received. "He didn't know what he was walking into."

Ode and his wife have no children.

The State Department had no ready explanation last night for Ode's reference in his letter to being "one of approximately 65 United States citizen members of the American Embassy in Tehran, Iran who have been held as hostages" since Nov. 4.

The State Department had said in recent weeks that 50 Americans were still being held. One possible explanation of the discrepancy on numbers was that when Ode wrote the letter to The Post on Dec. 26, he may have been unaware that 13 of the American hostages had been released.

Ode's sister said that Ode, in one letter, had described the temperature at the embassy as "terribly cold," with only two heaters operating.

"He's just sending out a desperate plea for help," said his sister. "They're not getting any news. So he doesn't know if they're (the U.S. government) doing anything to help or not."

In his letter to The Post, Ode wrote, "Since our captors do not permit us to receive news of any kind, either by radio, TV, newspapers or recent magazines, we have no idea of what is being done by the president, the secretary of state, by members of the Congress, or any other officials of the U.S. government to protect our basic human rights or our welfare as United States citizens.

"Our diplomatic immunity has consistently been ignored by our captors."

On Christmas, the day before Ode's letter was dated, Ode was seen by the Rev. William Sloane Coffin Jr., one of the three American clergyman who held Christmas services for the hostages. Coffin later said, "The oldest hostage, Mr. Ode, he seemed to be bearing up fairly well, but you know, there's tension in there."

Ode, who grew up in Michigan and served in the Navy in World War II, joined the State Department in 1947. His posts included Poland, England, Iceland, Italy, Canada, Liberia, Switzerland and West Germany. Since his retirement, he has had temporary assignments to Guyana and Jamaica before going to Tehran.

Among other letters received from hostages in recent days was one from Marine Cpl. William A. Gallegos, the embassy guard from Pueblo, Colo., who was interviewed in a controversial television broadcast last month.

The letter, printed yesterday by the Jersey Journal in Jersey City, N.J., was written to a 7-year-old second grader in Bayonne, N.J., who had sent Gallegos a Christmas card. Gallegos' letter thanked her for the card, saying, "You're so young; yet you understand our situation . . . I hope your prayers will soon bring us home."

Hazel Albin of North Little Rock, Ark., the mother of 52-year-old hostage Robert O. Blucker, said in a telephone interview last night that a letter from her son was received yesterday by a friend in Washington. In the letter, she said, her son expressed concern about belongings he had shipped to Tehran some time ago.

In Chicago, two persons who had mailed Christmas cards to a hostage, Sgt. Paul E. Lewis, a Marine guard, received replies from him yesterday. "On behalf of all the hostages in Tehran I would like to express our heartfelt thanks that you and so many other Americans have kept us in your thoughts and prayers," he wrote.