A man armed with three high-powered rifles was arrested today after he allegedly told police he was on his way to Washington to kill President Reagan, Secret Service agents said.

The agents said Isom Joseph Dean Jr., 27, of Sparks, Md., told them "he hated the president and that the president 'should be disposed of.' "

The Secret Service said Dean was arrested in nearby Towson, Md., on his way to Washington. Police said they found three scope-equipped rifles, an M-18 semiautomatic rifle, a handgun, and a pair of high-powered binoculars in the car he was driving.

Charged with threatening to assassinate the president, Dean was ordered held without bond.

In an affidavit, Secret Service agents said Dean told them he intended to kill the president and "planned to sit around and wait until the opportunity presented itself because the president always has to come out."

Police pulled Dean's car over in responding to a report of a car being driven by an unauthorized person.

Public Defender Fred Bennett told Rosenberg his client had been drinking and had taken the drug PCP before his arrest. Bennett said Dean, from Sparks, Md., had been hospitalized several times for psychiatric disorders.

Police spokesman E. Jay Miller quoted Dean as telling arresting officers: "You're lucky. If I hadn't had to stop for gas in Towson, you would not have gotten me. I would have killed the president."