Ramsey Clark, returning today from his mission to Iran, said government officials "should do their duty" if they believe they can prosecute him for violating President Carter's ban on travel to Iran.

Clark arrived at Kennedy Airport aboard an Air Force jet from Paris and at a news conference said U.S. Customs officials inspecting his baggage asked to borrow seven pamphlets containing copies of the 1906 Constitution of Iran, "an Op-Ed page of The New York Times . . . and a few other things." He said they told him the pamphlets would be returned after they had made copies.

Clark, a former attorney general, and nine other Americans traveled to Tehran two weeks ago and attended a conference there on past U.S. "crimes" against Iran during the reign of the deposed shah.

Before they left, Attorney General Benjamin Civiletti warned that violation of the travel ban, issued under the International Emergency Economic Powers Act, carried civil and criminal penalties including a maximum criminal penalty of 10 years in prison and a $50,000 fine.

Commenting on Carter's statement that it was his "inclination" to prosecute Clark and the others, Clark said, "I think President Carter should leave it up to the law and out of politics . . . Law and politics don't mix well."

Clark reacted angrily when a reporter brought up allegations that the Ayatollah Ruhollah Khomeini had revived the shah's secret police and asked Clark if he regretted having "cooperated with Iran."

"What are you talking about -- cooperating?" Clark asked, his face reddening. "Where did you get that idea?"

He said, he did not know whether reports about the return of secret police were true, adding, "I'm not here to defend what's happening in Iran."

But he expressed on regrets about his trip.

"I am glad I attended the conference where 12 out of 54 nations spoke out for freedom of the hostages," said Clark. "I had asked that there be release of them now, but there was a pause and I realized that the dispute can be resolved only through negotiation.

"While I thought of the 53 of our own being held captive, I also realized that the action came out of defiance of a small nation who had had 20,000 killed under the shah's regime.

"I do not think that 'our country right or wrong' holds true. We must follow the dictates of law. . . ."

Clark insisted that his trip was within his rights as a U.S. citizen. "When did we become fearful of dialogue, afraid to talk?" he asked.

"I am a free person and I speak my mind freely and I will continue to do so whereever I go," Clark said.

Clark said he did not see any of the 53 Americans who have been held hostage by militants since the U.S. Embassy in Tehran was seized Nov. 4. He said he had no indication when the hostages might be released.

"Each aggressive act by the United States prolongs the holding of the hostages," he warned.

The Treasury Department has been investigating the Clark trip. The Justice Department has said Civiletti will make the decision on whether to act against Clark.

Clark, a Texas-born lawyer who headed the Justice Department under President Johnson, was chosen by Carter last November as a special envoy to seek release of the hostages, but Iran denied him entry to the country. Clark subsequently criticized Carter's handling of the Iranian crisis.

In an interview with United Press International, Clark said the United States should start a congressional investigation into its past involvement in Iranian affairs to win the release of the hostages.