Frank A. Kerrigan, a 21-year member of the U.S. Capitol Police force, has been appointed its chief, congressional officials announced yesterday.

Kerrigan, 46, was chosen for the $77,000-a-year post after he came to the attention of Capitol Hill officials during the Iran-contra hearings when he directed complicated security measures for the highly publicized, nationally televised hearings that concluded last week.

"He was very much respected . . . and very capable in that highly pressured atmosphere with all the implied threats involved in the Iran-contra hearings. The protesters were removed immediately and Kerrigan supervised all that," said Patricia McNally, executive assistant to Henry K. Giugni, chairman of the U.S. Capitol Police Board, a three-member panel that selected Kerrigan.

Kerrigan, an inspector in the department's Senate division, succeeds James J. Cavino, who was appointed chief in August 1984 and resigned for personal reasons effective last Thursday. Cavino is being investigated by the General Accounting Office for possible misappropriation of funds after he approved spending $2,000 to produce a videotape of Washington as a farewell gift for a secretary.

"I never expected this job; it was a surprise. It's going to be a challenge but I'm looking forward to the challenge," Kerrigan, who joined the force as a patrol officer in 1966, said in an interview yesterday.

He played down his performance during the hearings, saying that security measures presented a logistical hassle fueled by press and public interest. Many witnesses, such as Marine Lt. Col. Oliver L. North, had their own security guards but "there were no major problems."

"The day that stands out, really, was the last day, when it was really over with. People kept calling me and telling me I was on TV and it was really something to see, but it really wasn't that exciting," Kerrigan said, adding that he escorted North and other witnesses in and out of hearings and made sure that they were not mobbed during breaks in testimony.

More interesting, Kerrigan said, were massive demonstrations at the Capitol during the Vietnam War and a three-month protest by farmers in the winter of 1979.

"That's one of the most interesting times I've ever had here -- the fact that the farmers and their tractors were actually camped out on the Mall. Most demonstrators are here and gone but that lasted for months," Kerrigan said. "The first couple of days were kind of tense with all the tractors and we were on foot but it changed. They got to know us by name and we got to know them by name."

A native of Parkersburg, W.Va., and the son of a police officer there, Kerrigan said that security for the 535 members of the House and Senate and their staffs is his top priority as chief of the 1,270-member force.

For example, in 1985 there were 178 threats against members or the Capitol itself, including 23 bomb threats. Since 1983, nearly 200 people have been arrested on weapons charges, officials said.

"Security is the most important aspect. Nowadays it's even more important with the advent of terrorism. That's become the main concern of everyone involved in law enforcement," he said. "I need to look Hill-wide and see what's in place and what needs to be in place."