D.C. police adjusted their calculation of the size of the crowd at the Lincoln Memorial Saturday and estimated yesterday that the rally was attended by 300,000 persons, instead of the 250,000 estimated earlier.

Police said the 250,000 figure released Saturday reflected only those actually assembled at the Lincoln Memorial when the size of the crowd there reached its peak. Factoring in what police described as a constant turnover in the crowd gave a better picture of the true size of the demonstration, officials said.

"At one point there were as many people going as there were coming," said Assistant D.C. Police Chief Marty Tapscott.

Officials noted that the 95-degree heat of the day forced many participants to cut short their stay or to take shelter in some shady spot outside of the boundaries of the area in which the 250,000 estimate was made.

Although police said their adjustment for comings and goings was necessarily based on an estimate, both D.C. and U.S. Park Police used a system of photographic measurement to calculate the number of persons at the Lincoln Memorial and along the flanks of the Reflecting Pool.

In essence the technique involves dividing the total area occupied by the crowd by the number of square feet occupied by the average person. Refinements include adjusting the area allotted to each person based on whether people are standing or sitting.

A spokeswoman for the National Park Service agreed that while the crowd at the Lincoln Memorial was estimated at its peak at 250,000, "in essence it was in excess of 250,000 because people were leaving and coming . . . all throughout the day."

She noted that even with the constant departures, enough people arrived to hold the crowd at 250,000 from 2 p.m. to 4 p.m.

In making their revised estimate, police said they took into account people who were on the Mall during the demonstration but far from the Lincoln Memorial, and those who at any given time during the rally were in the bus parking lots.

In other developments related to the march, police said they had accounted for all but 16 of the 125 persons reported missing on Saturday.

Police said they made no more than about three dozen arrests, including 19 on charges of vending without a license.

In one of the more bizarre incidents of the day, officials said an automobile plunged into the Tidal Basin after its driver got out to ask directions but left the motor running. No one was injured and the vehicle was retrieved.