A Prince George's County judge convicted 21 antiabortion protesters yesterday of trespassing at a Greenbelt abortion clinic in March, and 10 of the defendants were ordered jailed on contempt of court charges for refusing to pay their fines.

Four of the defendants, who had prior convictions for trespassing during demonstrations at abortion clinics, were sentenced to 10 days in jail. Six others were sentenced to five days in jail.

District Court Judge Irving Fisher, who presided over the three-day trial of the group, imposed fines of $400 for the remaining defendants who had prior convictions and $150 for first-time offenders. Trespassing, a misdemeanor, carries a maximum penalty of a $500 fine.

Malia Brock, who lives in Woodbridge, Va., told the judge that she couldn't pay her $150 fine because she went to the clinic to save lives. "I feel I am not guilty of anything," she said.

Her mother, May Brock, followed that lead. She told Fisher that she also would not pay her $150 fine. "I cannot do anything less than my daughter, your honor," she said.

In imposing the jail sentences, Fisher said the proceeding started as a trespassing case, "but now it's reached different proportions."

Prince George's County police arrested the demonstrators March 30 when they refused to move from the entrance of the Metropolitan Family Clinic at 5915 Greenbelt Rd.

The demonstrators, representing the antiabortion Non-Violent Action Project, a loose affiliation of groups opposed to abortion, had planned to block the entrance to the clinic and place members in the waiting rooms to talk to women about alternatives to abortion.

Thomas Herley of New York City, who refused to pay a $400 fine, told Fisher that most of the defendants knew what the penalty would be when they tried to block the doors of the clinic. "We don't regard this as a joke. We have done it because we think it's right," Herley said.

Assistant State's Attorney Mary I. Scherstrom told Fisher during closing arguments that the defendants' actions were forms of vigilantism. "It's a contempt for the way our system of government is organized," Scherstrom said. "We don't have to go out and break the law to change the law."