French President Valery Giscard d'Estaing tonight delivered his first detailed defense against accusations that he received large gifts of diamonds from deposed Central African emperor Jean-Bedel Bokassa.

In a televised reelection campaign appearance with a panel of journalists, Giscard admitted receiving the gems, as he has implicitly done before. But, he said, all that he took form Bokassa were gifts of ivory and diamonds that did not exceed the average value of presents that the former emperor used to give to any visiting chief of state as samples of the production of his national diamond-cutting and ivory-polishing facilities.

As soon as Bokassa was overthrown in September 1979, Giscard said, the diamonds the French president received were sold for the benefit of Central African charities, notably for the former French colony's Red Cross.

The campaign against him on the subject, Giscard said, was "base and cowardly." He said his detractors had relied on documents and rumors spread by "poor devils" in the service of a deposed chief of state thirsting for revenge.

Giscard said he would have answered questions about it sooner if he had been asked about it the same way he was tonight. His questioner was extremely apologetic, recalled that the president had said the time came. Is this the moment, the questioner asked.

The last time a journalist referred to it, even less directly, Giscard got obvciously angry, said, "I beg your pardon," and changed the subject.

The president dealt with another latent campaign issue that has been hurting his chances -- the prospect that if he is reelected to a second seven-year term, that would make a total of 14 years in power. If reelected, Giscard said, he would not run for a third seven years.

He said he actually favors amending the constitution to reduce the term by a year and to make it renewable only once for a total of 12 years, but he did not indicate that he thought that should apply to him.

Turning to the accusation that he has been a monarchical president, he noted that monarchy is inherited, hardly the situation of a man running for reelection. As for royal protocol in the presidential palace, he said he had merely continued previous practices.

Those who make the accusation that he acts like an uncrowded monarch, he said, want a weak president.

"Those who want a weak power in France should not vote for me," he said.

But the issue of the diamonds and Giscard's complicated relationship with Bokassa is not going away that simply. Writer Roger Delpey, undoubtedly the principal "poor devil" Giscard had in mind, revealed today that a particularly convincing death threat is holding up, perhaps for good, publication of a potentially explosive set of documents purporting to show the true nature of the Giscard-Bokassa ties.

Delpey has been working for more than a year on writing Bokassa's side of the story in the face of intensive harassment that has included a stretch in jail on questionable charges.

The latest obstacle placed in his path, Delpey said today, was a typewritten death threat delivered Feb. 11 against a Swiss man and his family whom the writer was counting on to deliver the portion of his book that could hurt Giscard's reelection chances most -- a sheaf of 187 documents that were given to Delpey by Bokassa and placed in safekeeping with the Swiss man in a secret hiding place. The documents, Delpey has said, include correspondence between Bokassa and Giscard.

The apparently frightened, unnamed Swiss, has been refusing since he got the threat to return the documents for inclusion in Delpey's forthcoming book. The recipient of the threat took it seriously because, Delpey's friends say, only an organization such as an intelligence service with sophisticated equipment to trace telephone calls and the like could have identified him and found his address.