OFFUTT AIR FORCE BASE, Neb. (AP) _ About 240 people, including members of the Great Peace March, were detained briefly Sunday for trespassing on Offut Air Force Base during a peaceful protest by about 500 demonstrators, authorities said.

''It was inevitable. They were going to come out to Offutt and protest (against nuclear arms) and they did,'' said Lt. Col. Ralph Tosti, public affairs officers for Offutt, headquarters of the Strategic Air Command.

Tosti said about 240 people crossed a white line at the Bellevue Gate, which constitutes unauthorized entry into the military base located just south of Omaha.

''I thought it went very well,'' he said. ''They did a good job of letting us know which gate they were going to be at and what time. It was all very peaceful.''

As they crossed the line, military authorities took the protesters to waiting buses for transportation to a processing center on the base.

Observers said about 60 security officers stood three deep to take the protesters into custody. Several of the protesters had to be carried onto the buses but most went without incident, Tosti said.

Tosti said the protesters were photographed, fingerprinted and asked to fill out questionnaires listing their names, addresses and group affiliation.

Then the protesters were issued ''ban and bar'' letters, which are issued to first-time trespassers at the base, and were transported back to the gate for release, Tosti said.

If a person with a letter enters a second time without permission, he or she could be subject to federal prosecution, Tosti said.

Offutt has been the target of peace protests since about 1979, Tosti said. He also said between 50 and 75 pro-SAC demonstrators, members of a group called Nebraska Conservatives for Freedom, showed up to demonstrate against the peace protest.

He said many members of that group, like the peace advocates, carried signs and shouted their opinions.

Tosti said several of the protesters also gave Offutt officials seeds, corn and baskets of fruit. ''I know part of their cause was 'Farms Not Arms,' so I think the seeds and corn were along that line,'' he said.

Tosti didn't know how many of those detained were participants in the Great Peace March, which currently is camping at nearby Lake Manawa in Iowa.

The peace marchers set out from Los Angeles on March 1 to walk across the nation promoting disarmament. They hope to arrive Nov. 15 in Washington, D.C.