An alleged officer of the Bulgarian secret police who was caught carrying secret U.S. data on nuclear energy was arraigned today on espionage charges and held without bail pending a hearing Oct. 4.

In the complaint filed by the government, Penyu B. Kostadinov, 41, was accused of taking a classified document called the "Report on Inspection of the Nevada Operations Office." He also was accused of giving a confidential U.S. source a list of secret documents he wanted to obtain.

"The offense for which Mr. Kostadinov was arrested last night was about the most serious crime that exists under federal law," Assistant U.S. Attorney Ruth Wedgewood told U.S. Magistrate Nina Gershon.

The government's confidential source said Kostadinov had asked him several times for documents and information since May, 1981.

Authorities said he got the material with the aid of an unidentified American graduate student he had befriended. The student was working for the FBI.

If convicted, the Bulgarian could face a sentence of up to life in prison or death, but the death penalty has not been invoked here since the law was written in 1954. Kostadinov does not have diplomatic immunity, the FBI and State Department have said.

In Sofia, meanwhile, the official Bulgarian news agency BTA reported that Bulgaria has lodged a sharp protest with the U.S. ambassador there and called for Kostadinov's immediate release.

U.S. Ambassador Robert L. Barry was summoned to the Foreign Ministry in Sofia and told that the charges are "groundless" and "can be interpreted only as a crude and premeditated provocation against Bulgaria."

Defense attorney Stanley Singer said here that his client should not be held without bail because "the Bulgarian ambassador is willing to give his personal assurance to guarantee his appearance."

Kostadinov was arrested at 7:15 p.m. Friday as he left a dinner in Manhattan with the American graduate student. During the dinner the student gave Kostadinov the documents, the FBI said.

FBI agents posing as diners watched the exchange. As the Bulgarian left the building he was arrested by 20 FBI agents who were waiting for him, FBI spokesman Joseph Valiquette said.

Valiquette said agents recovered the "very highly classified" material.

Although Kostadinov was listed as an assistant counselor for the Bulgarian commercial office, Valiquette said Kostadinov was a member of DS (Durzhavna Sigurnost), the Bulgarian state security organization that collects foreign intelligence.

An FBI official said DS is the Bulgarian equivalent of, and is overseen by, the Soviet KGB.

Valiquette described Kostadinov, who has been in the United States since 1979, as a "full-fledged intelligence officer" believed to carry a high rank in the Bulgarian spy agency.

State Department spokesman Anita Stockman declined comment on Kostadinov's arrest because his liability for prosecution made it a criminal, not diplomatic, matter.