President Reagan moved quickly yesterday to disassociate his administration from a CIA manual advocating political assassinations in Nicaragua, ordering an investigation of "the possibility of improper conduct" within the CIA.

Senior administration officials described the manual privately as "the work of an overzealous free-lance," an independent employe under contract to the CIA.

Reagan acted as leading Democrats, sensing a potentially hot political issue that could embarrass him in Sunday's foreign policy debate, chorused denunciations of the manual, the CIA and the administration's policy of aiding rebels against Nicaragua's leftist Sandinista government. House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) called for the resignation of CIA Director William J. Casey.

"The content and the tactics contained in this manual would never have been condoned by the president or the national security community," White House spokesman Larry Speakes said in New York, where Reagan attended the Alfred E. Smith Memorial Dinner last night.

Before leaving Washington, Speakes issued a statement in which Reagan asked Casey to have the CIA inspector general "investigate the possibility of improper conduct on the part of employes of the CIA in regard to the publication of a manual for the Nicaraguan democratic resistance forces." He also requested a probe by the Intelligence Oversight Board, which consists of three members he appointed.

"The administration has not advocated or condoned political assassination or any other attacks on civilians, nor will we," Speakes said.

The manual originally was obtained by the Associated Press.

Administration officials, speaking on condition they not be named, said the 90-page booklet apparently was prepared by a low-level contract employee with Vietnam experience, and had never been cleared for publication. They said they have "a definite suspect" in mind as the author, but would not say whether the person still is under contract.

Although officials said the manual was a draft, the head of the largest Nicaraguan rebel group said he had received a copy of it. Adolfo Calero, head of the Nicaraguan Democratic Force (FDN) which is based in Honduras, said the document was given to him not by the CIA but by a supporter he declined to name.

Some sections, he said, had been ignored. "It talks about terrorism, which is something we haven't done," Calero said.

The administration officials acknowledged that the manual was "clearly against the law" as outlined by Reagan in a Dec. 4, 1981, executive order that banned political assassinations.

The pamphlet, entitled "Psychological Operations in Guerrilla Warfare," recommends that Nicaraguan rebels use "selective violence" to "neutralize" prominent members of the Sandinista government, terms intelligence sources said can mean murder or permanent disabling.

It says professional criminals can be hired to do "selective jobs" such as creating "martyrs" through staged violent demonstrations in which rebel backers are killed. The document also advises guerrillas to be polite and helpful to rural Nicaraguans to build support.

Democratic presidential nominee Walter F. Mondale said in a Cable News Network television interview yesterday that the manual could "deprive the United States of its moral authority" and was further proof that "the whole illegal, covert war in Nicaragua has been counterproductive."

His running mate, Geraldine A. Ferraro, said the manual was "totally contrary to our basic values . . . . Anyone who believes for one minute that refining murder techniques of Central Americans will advance our national interests is gravely mistaken."

O'Neill told reporters that the manual was "nothing short of outrageous" and said Casey "owes the Congress and the American people an immediate explanation." He added, "I want him out of there."

His call was echoed by Rep. James N. Shannon (D-Mass.).

Sen. Barry Goldwater (R-Ariz.), chairman of the Senate Intelligence Committee, demanded "a complete report" on the manual's history from the CIA, and staff members said later that the agency is expected to brief the committee Monday.

Reagan's senior staff members discussed the manual with the president on Wednesday after a story about it was published in The New York Times. The president's political advisers expressed concern that Reagan act quickly to repudiate the document before Sunday's televised debate with Mondale.

Administration officials said national security adviser Robert C. McFarlane gave his report to the president and told him that he believed the manual was the work of a "free-lancer" operating under contract to the CIA.

Officials declined to say whether the results of the probes would be made public or whether they would be given to the president before the Nov. 6 election.

CIA spokeswoman Patti Voltz said the CIA would cooperate fully with any investigation. She said, however, that inspector general John Stein would disqualify himself because he was the CIA's deputy director of operations from 1981 to July 1 and, therefore, was in charge of the administration's covert program in Nicaragua. The probe will be handled by the CIA's deputy inspector general, whom Voltz declined to name.

Another CIA-sponsored manual, in the form of a comic book, surfaced last summer in Nicaragua. It described how to make Molotov cocktails, pour sugar in gasoline tanks, disrupt communications and otherwise harass the Sandinista government. Sources within the FDN rebel group confirmed the book had been provided by CIA contacts. But it did not specifically advocate or demonstrate any "neutralization" techniques, in contrast to the psychological warfare manual.