Simas Kudirka, who once served four years in a Soviet concentration camp for attempting to escape to the U.S. before winning asylum here, yesterday walked out of D.C. Superior Court a free man, acquitted of a charge of protesting too close to the Soviet embassy last summer.

Kudirka is the Lithuanian dissident whose forcible return to the Soviets in 1970 after the United States refused his request on the high seas for political asylum caused a national furor. He left the courtroom pleased that the American judge had been more receptive to his case than the Soviet jurist who convicted him a decade ago.

"It is so unbelievably different," Kudirka said, "it is different like night and day. The Soviet Union may be a union for some, but for us it is chains."

Kudirka and 17 other anti-Soviet demonstrators were arrested last July 18 after they swooped down on the Soviet embassy on 16th Street NW, stopped traffic and attempted to handcuff themselves to the embassy gates to protest the opening of the Moscow Olympics. The demonstrators were arrested and charged with violating a city law that prohibits such protests within 500 feet of foreign embassies.

Judge Nicholas S. Nunzio found that in Kudirka's case, as well as in those of nine other protestors, U.S. Secret Service officers had failed to adequately warn the demonstrators they were violating the 500-foot rule as required under law. Three others were found guilty and sentenced to time served, which amounted to the day they had spent in jail last summer. Charges against five others were dropped.

The trail that led Kudirka from the confines of a Soviet prison to a day in the D.C. jail began with his fateful 1970 leap into prominence from a Soviet fishing trawler off Martha's Vineyard, Mass., onto the decks of a U.S. Coast Guard cutter. He begged for asylum, but was rejected. He was returned to Russia and spent time in prison before he was finally released when it was discovered that he was entitled to U.S. citizenship because his mother was American-born.

Government prosecutors unfurled several large anti-Soviet banners in the D.C. courtroom as evidence and carted in a box full of silver chains and locks used by the demonstrators. Still, the atmosphere in the crowded courtroom was friendly. Kudirka was all smiles and nodding his head proudly as a U.S. Secret Service officer testified against him.

Assistant U.S. Attorney Michael L. Rankin argued that in the massive confusion surrounding the arrests of the demonstrators, "it was not plausible that the officers could go up to each and tell them to stop" as required by the law. The dissidents' lawyer, Ernest C. Raskauskas Jr., argued that the officers' arrests had been improper.

After the hearing, Kudirka, who now works for a Lithuanian rights organization in New York, approached one of the three Secret Service officers who had testified against him. "My name is Simas Kudirka," he said in halting English, "I'm very, very sorry you not in so nice situation. You not informed about Russian empire. We must tell the truth." Court marshals and Kudirka's supporters quickly hustled the outspoken dissident down the corridor lest he be again arrested.