A 29-year-old man was arrested after a brief chase at National Airport last night when he allegedly tried to board an Eastern Airlines flight to Miami with a .45-caliber automatic pistol concealed inside a portable radio, authorities said.

An airport police official said the man fled when the weapon was detected by an X-ray machine at a security checkpoint.

He was chased by two officers and arrested about 100 yards away inside the terminal.

Leon Gaskins, 29, of no fixed address, was being held last night at the Fairfax County Detention Center pending arraignment this morning before a federal magistrate on a charge of attempting to board an aircraft with a dangerous weapon.

The official, who asked not to be identified, said Gaskins was being sought on a warrant in Prince George's County for allegedly failing to appear in court on a criminal charge there.

The official said Gaskins was late for his flight when he approached the security checkpoint with his luggage about 9:25 p.m. The X-ray device activated a silent alarm at the airport police station.

Police opened a bag and found the radio in which the pistol was concealed, the official said. When questioned by police, Gaskins said he had been paid by someone else to take the weapon aboard the plane, the official said.