In a courtroom packed with handpicked official spectators, Soviet dissident leader Yuri Orlov went on trial here yesterday and learned for the first time that he was charged with selling "slanderous material" to the West. As the trial began, the court denied Orlov the right to call witnesses in his own defense.

The Kremlin thus opened, after long preparation, the first of an expected series of trials of dissident leaders aimed at throttling the human rights movement here by labeling its activists common criminals, and warning the West in general and President Carter specifically, not to meddle in "internal affairs" or give them support.

Orlov, 52, looked composed and was smiling when he was brought into the court, according to his wife, Irina, whom he had not seen or spoken with since Feb. 10, 1977, when he was arrested by the secret police and jailed.

Irina Orlov and her two step sons Dmitri, 25, and Alexander, 23, were the only outsiders allowed into the small courtroom.

The courtroom was packed with specially admitted official spectators, whom Tass, the government news agency, called "representatives of the public."

Western reporters and Richard E. Combs, an American diplomat assigned by the U.S. embassy to observe the trial, were barred from the courtroom, as were numerous friends of Orlov, including Nobel Peace Prize winner Andrei Sakharov, also a leading dissident.

In Washington, the State Department expressed its concern and said it regretted that the Kremlin refused to agree to the American request to have an observer.

"We are obviously concerned about the trials of those attempting to monitor compliance with the Helsinki final act," State Department spokesman Hodding Carter said.

Orlov, who founded a group to monitor Soviet compliance with the human rights guarantees of the Helsinki agreement on European security and cooperation signed by the Kremlin in 1975, told the court he was not guilty.

"I don't plead guilty because I don't understand the accusation," Orlov's wife quoted her husband as saying to the presiding judge, Valentina Lubentsova, who has handled other dissident cases.

Mrs. Orlov was not allowed to take any notes of the proceedings, but reported her impressions fresh from the courtroom to a group of other dissidents, family friends and Western correspondents gathered outside. She said the indictment ran to 40 pages of accusations.

She recounted this exchange between the prosecutor and her husband:

"Do you admit you are guilty?"

"I refuse to take part in these proceedings if I cannot explain my motives."

Police outside the courtroom showed extreme nervousness about tape recorders, once preventing an American reporter from taping Mrs. Orlov and later searching Irina, Ginzburg, wife of another jailed dissident, looking for a tape. As the court session ended, guards scuffled with the Orlov sons, reportedly confiscating tape recorders.

A major objective in the Kremlin's moves is to break dissident contacts with Western diplomats and journalists, whose reports have been a major factor in shaping the outcry in the United States and elsewhere over human rights repression here. Those reports also pose further problems for the leadership because they often are broadcast back into the U.S.S.R.

Tass, in unusually quick reports on the trial, reflected this theme. In one brief account, declared that Orlov "admitted . . . testifying to making and distributing in 1973-77 documents which contain fabrications, smearing the Soviet state and social system . . . (which) were systematically handed over by him to correspondents of capitalist countries, accredited in Moscow, and these were also addressed by him to governments of foreign states and international organizations."

The court papers do not mention the name of Orlov's group called the Moscow-Helsinki Monitoring Group. Besides Orlov, the papers name Alexander Ginzburg and Anatoli Scharansky, who were also members of the group and are jailed awaiting similar trials.

A key element in the Kremlin's attack on the dissidents is the official line that they are common criminals who have nothing to do with the Helsinki accords.

The prosecution will center on reports circulated by the group of alleged human rights abuses. Under Soviet law, according to lawyers, it is not a crime to criticize the basis of the Soviet system or to express dissenting views. But this becomes a crime if a court holds that the purpose of such criticism is to weaken or subvert the Soviet state.

The prosecutor put into evidence as criminally slanderous several reports of the monitoring group, including one dealing with a village outside Moscow called Ilyinka, filled with Jews who allegedly are trying unsuccessfully to emigrate; one on alleged repression of Pentecostalist religious believers; and one describing how some improverished Soviet workers sought to emigrate to improve their lot in life.

Orlov, in a long statement of his motives and actions, admitted aiding in the writing and dissemination of these reports. "Yes, I wrote or took part, but that's no crime," his wife quoted him as saying. "My motives were humanitarian. I was not trying to subvert the state."

Judge Lubentsova, according to Mrs. Orlov, said the state had a film showing that the citizens of Ilyinka are happy with their life, so she considered the report slanderous. The judge reportedly denied Orlov's request to view the film.

Special correspondent Raymond Snoddy reported from London:

As Orlov went on trial in Moscow, a British lawyer supported by Soviet dissidents from the United States and France put forward here the evidence for the defense that they say will be barred by the court in Moscow.

Exiled writer Andrei Amalrik and physicist Valentin Hurchin flew to London from the United States to testify on Orlov's behalf. Maj. Gen. Pyotr Grigorenko testified by telephone from New York and Sakharov's wife, Elena Bonner, sent a tape recording.

The theme of their testimony was that Orlov has always been truthful and that the Soviet dissident groups had operated openly because they felt what they were doing was legal.