Convicted Soviet spy Robert Thompson was exchanged yesterday for American student Alan Van Norman, who was freed from an East German prison to complete a three-way international swap of prisoners.

"The psychological stress was quite high," Van Norman, 22, from Windom, Minn., told reporters in West Berlin after his release. He said he was not mistreated physically during nine months' captivity in five different prisons, but he added: "I was put in solitary confinement when I did not answer the questions asked me" during three months of "rough interviews."

Van Norman was improsoned last August for trying to help East Germans escape to the West.

Thompson, 43, arrived from New York earlier in the day to finish the transaction that also included Van Norman and Israeli pilot Miron Mareus, 24, who was released last month by Mozambique.

It was the first major East-West exchange of prisoners in West Berlin since U2 pilot Frances Gary Powers was exchanged for Soviet agent Rudolf Abel in 1962.

Thompson, a U.S. Air Force clerk in 1965 when he was convicted of passing secrets to the Russians, was released Sunday from Lewisburg Federal Penitentiary in Pennsylvania after serving 13 years of a 30-year term for espionage. He was accompanied to Europe by East German attorney Wolfgang Vogel, who also helped arrange the prisoner swap involving Powers in 1962.

The exchange took place behind closed doors in the U.S. mission here. An American statement said Thompson and Vogel then crossed into East Germany, a country that Thompson said has "plenty of freedom."

Van Norman said he had no idea he was about to be freed.

"My lawyer's wife picked me up from the company of state security guards and brought me to West Berlin. I cannot remember whether I shook hands with Thompson," said the student, who looked pale but healthy and was dressed in a three-piece pin-striped suit.

Van Norman said he tried to arrange for the escape of Dr. Juergen Grafe, his wife and son, in the trunk of his car "for personal reasons - no girl friend, not for money." He said the mission was the idea of a group he met while he was studying in Europe last summer.

The physical treatment wasn't particularly fantastic, but I lived through it," he said. But the psychological strain is quite high.

"The interrogators keep telling you over and over again you're lying, that you're not saying all you know. Then they throw propaganda at you every day about that a terible country you come from."

Thompson, before boarding his plane for New York, told reporters that "Robert Thompson" was not his real name, but he refused to confirm reports he was a major in the Soviet secret police.