F. Keith Adkinson, a prominent Washington lawyer and former Reagan nominee to the Federal Trade Commission, was arrested yesterday and charged with assault after his 8-year-old son told Montgomery County police that his father held a gun to his head and made him watch violent movies, county police said.

Police said they took action in response to a complaint from the boy, who telephoned Montgomery police April 23 and said that during a weekend visit to his father April 5 and 6, Adkinson forced him to watch the violent films, characterized by one police officer as "horror movies." Adkinson, 41, is separated from his wife who lives with their son in Harpers Ferry, W.Va.

The boy, who was not identified because of his age, told police that Adkinson also rolled a grenade into the room where he was watching television, according to Cpl. Phil Caswell, a county police spokesman. Police said they did not know whether the grenade was live, or whether the gun was loaded.

"I categorically deny all of those charges," Adkinson said last night after his release. " . . . I didn't hold a gun to his head . . . . I don't even spank the boy when he deserves it." He called the charges an "outgrowth of what evidently has been a very bitter domestic dispute" involving his wife. She could not be reached for comment.

Early yesterday, county police searched a house in Wheaton where Adkinson had been staying, and said they found an automatic rifle, handguns and grenades in that search.

A fugitive warrant was then obtained for Adkinson's arrest. He was charged with assault and possession of explosives.

Maryland State Police took Adkinson into custody in Frederick County. He was driving a white Jaguar south on Rte. I-270 with a companion, police said, and a loaded .38-caliber handgun and fireworks were found in the car. Police later charged Adkinson and his companion, identified by police as Cathy Mayer, 33, with illegal possession of a handgun.

Adkinson was then turned over to Montgomery authorities, and he was released on personal recognizance last night. Mayer was released on personal recognizance by police.

Adkinson said he had no automatic weapons. He said a flare and a grenade simulator, which makes noise but does no damage, accounting for the explosives-possession charges. He said he has a gun permit in Alabama and holds a deputy sheriff's commission in Tennessee.

Adkinson has played an active role in Washington-area politics. In the early 1970s, he ran for a Virginia House of Delegates seat from Alexandria. In 1979, he served as a staff member with the U.S. Senate permanent subcommittee on investigations. In 1980, he was national chairman of Democrats for Reagan. In 1981, he was tapped to serve as an FTC commissioner.

Reagan withdrew the nomination at Adkinson's request after it became apparent that there was little chance of his being confirmed by the Senate Commerce Committee. Committee members apparently sought to block the nomination in light of allegations that Adkinson acted improperly when he signed a book and movie contract with a government witness while working on the investigation subcommittee.

Before his nomination, Adkinson had been with the Washington-based law firm of Seyfarth, Shaw, Fairweather and Geraldson, and had served as chief assistant to Sen. Sam Nunn (D-Ga.) during the late 1970s.

Adkinson gained prominence representing celebrities such as David Soul, an actor who starred in the popular Starsky and Hutch television series, and Cathy Stone, a singer who claims to be the illegitimate daughter of country singer Hank Williams.

Several persons who have worked with Adkinson said he cultivated an image as a hard-edged attorney who liked a flamboyant life style and wore cowboy boots with his three-piece suits.

At the same time, he has a good relationship with his son, said Carol Cummings, a longtime family friend.

"They have an exceptionally good relationship, a very substantial relationship," said Cummings, who also described Adkinson as "a very friendly person."