Five years after he first considered running for governor of Maryland, Robert A. Pascal today made his candidacy official, opening his campaign by accusing Gov. Harry Hughes of costing Marylanders $24 million because of his delay in deciding to build a new prison.

"The current administration sat on its hands for three years," Pascal said. "Three years ago we could have built an 890-bed prison for $26 million. Now, a 720-bed facility will cost $50 million and take four years to build."

The prison issue, long a favorite of the Anne Arundel County executive, provided a centerpiece for Pascal's daylong theme: "The current administration has been drifting aimlessly for three years. I offer a clear choice."

With his wife, Nancy, by his side, Pascal, 47, made his announcement three times, in a senior citizens center named for him, in an East Baltimore restaurant and finally in a Silver Spring hotel. Throughout the day, Pascal focused on Hughes' "lack of leadership" and the problems of crime, juvenile delinquency and the aging.

His announcement was greeted with relief from Republican officials who, after suffering quietly in 1978 when Pascal looked like a candidate but never became one, had wondered when he would issue his official challenge to Hughes.

Pascal lingered long after his first speech, in front of about 200 senior citizens, shaking every hand in sight after promising to name a cabinet level officer whose sole responsibility would be problems of the aging.

In East Baltimore, Pascal announced that, at the request of Sen. Charles McC. Mathias, Rep. Marjorie Holt and himself, Navy Department officials again would look at the unemployment problem there.

Painting himself as the workingman's friend, Pascal addressed another group of about 200 that included many Democratic labor leaders and some staffers from Baltimore Mayor William Donald Schaefer's office.

In that speech, Pascal reached for Schaefer's coattails, saying that as governor he would continue state funding for Schaefer's "successful efforts to revitalize Baltimore city."

"If I thought Don Schaefer was going to run, I would not be in this race," Pascal said. "Because if he ran, he would surely win."

Meanwhile, Hughes, in an Annapolis press conference, said he expects to seek the endorsement of Schaefer, who has critized him in the past for lack of support for Baltimore.

Pascal, who concedes he is relatively unknown outside his home county and Baltimore, said he plans to campaign extensively in the Washington area.

"Why should the people in the Washington suburbs know the Anne Arundel executive?" Pascal said. "My only claim to fame down here is that I played in the same backfield with Sonny Jurgensen and went to school with Lefty Driesell."

The county executive may try to improve his lack of recognition by choosing a running mate from the Washington area. Baltimore County State's Attorney Sandy O'Connor, once thought to be Pascal's first choice, was at the Glen Burnie announcement, but said she expects to run for reelection.

Pascal, who calls himself "a fiscal conservative who cares about people," carefully avoided tying himself to the Republican administration in Washington. Asked if he would like to see President Reagan in Maryland campaigning as he did last year for Marshall Coleman in Virginia, Pascal took a swipe at the president.

"I would like to see him in Maryland tomorrow so I could take him to Bethlehem Steel and to the Navy yards so he could understand that we need to get the people there back working," Pascal said.