D.C. City Council member Marion Barry last night appeared to pull more securely into the lead in the cliffhanger Democratic mayoral election primary here as crucial absentee and challenged ballots were slowly counted by District election officials.

With these votes counted from 83 of the city's 137 precinets. Barry campaign workers said that 409 were for Barry, compared to 297 for incumbent Mayor Walter E. Washington and 279 for City Council Chairman Sterling Tucker. No official figures were available early last night.

The unofficial figures widened the 1.188-vote gap between Barry and Tucker already counted from the Sept. 12 primary. In this early count, Barry received 30,589 votes (about 34.5 percent of the citywide total) and Tucker got 29,401. or 33 percent.

Washington trailed both the other candidates in last week's count with 27,798 votes (31 percent).

The counting of absentee ballots and challenged ballots has become decisive since their total - about 3,000 ballots - is greater than the vote spread among the three candidates.

The outcome in two other city elections - the Ward 5 and Ward 6 council races - also is clouded by narrow vote margins between the top runners, and must await the final tally of absentee and challenged ballots.

Patricia Rice Press, who is trailing Ward 6 incumbent council member Nadine Winter by 92 votes, told the D.C. Board of Elections and Ethics yesterday that she may demand a recount.

The board of elections was scheduled to tabulate the estimated 3,000 challenged and absentee ballots Tuesday, but postponed the count until yesterday because election workers had skipped a preliminary procedural step for detecting and challenged voters.

Yesterday's count was scheduled to begin at noon, but did not get under way until 1:30 p.m. because of procedural delays. More than two dozen workers labored through the afternoon on the tedious task of opening envelopes containing about 2,400 absentee and challenged ballots and feeding them into electronic counting machines.

Elections administrator Mary Rodgers stopped the envelope opening process at about 5 p.m., citing exhaustion of many of the employes who had been working for the last 15 to 20 days without a day off. The remaining 500 to 600 absentee and challenged ballots, plus a small but undetermined number of other ballots, will be counted today.

The ballots counted yesterday were to be fed into the city's SHARE computer last night for an official citywide total as of yesterday's count.

Barry campaign workers noticed yesterday that a number - perhaps dozens - of challenged and absentee ballots were not being "read" by the electronic counting machines, apparently because the ballots had been marked with blue ball point pens and certain kinds of pencils. Election board technicians said the scanning devices in the machines often do not "read" or recorgnize such marks and were treating the ballots as if they were blank.

Barry campaign observer Phill Ogilvie asked that the ballots be handcount today. Elections administrator Rodgers agreed. Ogilvie also expressed concern that large numbers of ballots cast during the regular voting on Sept. 12 also may have been marked with ball point pens and could have been rejected as unread. He asked the board to furnish a citywide total of unread ballots.

Voting booths in the precincts were supposed to have been equiped with black felt tip pens or special pencils whose marks are recorded by the counting machines. Ogilvie and other campaign workers, however, said they saw ball point pens in some precincts.

Many of the "unread" ballots spotted yesterday apparently were absentee ballots filled out by voters who were away from the city or physically incapacitated during the Sept. 12 primary and who inadvertently used ball point pens.