WASHINGTON (AP) _ The Reagan administration filed ''strong protests'' with Soviet officials Saturday over the KGB's detention of an American reporter for allegedly receiving classified information.

The State Department said U.S. officials were ''making every effort to gain access'' to Nicholas Daniloff, Moscow correspondent for U.S. News & World Report, and that grounds for the KGB's action were ''contrived.''

Henry Trewhitt, the weekly magazine's deputy managing editor for international affairs, said Daniloff had been arrested but that the KGB had not filed charges.

''We vehemently reject any suggestion that he was engaged in any improper activities. This man has been framed,'' Trewhitt said at a news conference at the magazine's headquarters here. ''We demand his immediate release ... We hope that this outrageous set of circumstances will come to a speedy close.''

A statement released by the State Department and the White House said, ''Our embassy in Moscow has reported that Daniloff has been detained, allegedly for accepting clasified materials from a Soviet citizen.

''We are currently making every effort to gain access to Daniloff and to secure his release from custody. Regrettably, Soviet authorities have thus far not allowed us access.

''Based on the information we have, however, it is clear that the grounds on which he has been detained are contrived.

''We have thus launched strong protests at high levels here and in Moscow in which we have rejected any suggestion that Daniloff may have been engaged in activities incompatible with his status as a journalist and demanded his immediate release,'' the statement said.

Trewhitt said the purpose of the Soviet action appeared to be to intimidate American journalists and said he could not discount speculation that it was in retaliation for the Aug. 23 arrest of a Soviet U.N. employee, Gennady F. Zakharov, on spy charges in New York.

Trewhitt also said the State Department seemed to be making ''maximum effort'' to win Daniloff's release. But he said Soviet officials had not responded to requests for an explanation of the incident.

Daniloff's tenure as Moscow correspondent was scheduled to end on Sept. 8, Trewhitt said. He said Daniloff planned to stay in the Soviet Union for another month or so to do research for a book on his family's Russian roots.

Asked if the detention would change the magazine's plans to remain in Moscow, Trewhitt replied, ''We intend to pursue a bureau in Moscow and leave Jeff Trimble (Daniloff's successor) there.''

Daniloff, 52, was born in Paris and holds degrees from Harvard and Oxford. He began his journalism career in 1956 as a copyboy at The Washington Post and joined United Press International in London in 1959. He was assistant foreign editor of the Post from 1965 to 1966, and rejoined UPI in 1966.

Daniloff came to U.S. News in November 1980 and five months later was assigned to the Moscow bureau. Trewhitt said he thought Daniloff, who is fluent in Russian, had reported well and fairly on the Soviet Union.