The Metro subway system's new Orange Line stations were used by more than 21,000 passengers on Monday, far more than expected, according to Metro officials.

Statistics released yesterday by the transit authority show that 21,417 riders got on or off Monday at the four stations along the 9.1-mile Orange Line extension. The stations at East and West Falls Church, Dunn Loring and Vienna opened on Saturday.

Metro officials initially had estimated Monday's ridership at 14,000. They had predicted that daily ridership would rise to 19,000 after Metrobus service to the new stations begins on June 22 and had said they expected the total to reach 24,000 a day by the fall.

On Monday, officials said, a larger number of passengers took the subway during the middle of the day and the evening than had been expected. "We had a lot more off-peak ridership," said Robert A. Pickett, Metro's assistant planning director.

If the trend continues, Pickett said, the new stations will soon exceed their fall ridership target.

Ridership at the new stations increased slightly yesterday during the morning rush hour, with 7,078 passengers boarding trains compared with 6,634 Monday. Statistics were not available for the afternoon rush hour or for nonrush-hour periods.

Parking lots at the new stations also were used more heavily yesterday. By 10 a.m., officials said, the small lot at East Falls Church was 99 percent filled and the large Vienna lots were 84 percent occupied. More than half the spaces were taken at Dunn Loring and West Falls Church.

Transit officials have predicted that within a few months the lots at East and West Falls Church and Vienna will fill up regularly on weekdays. Spaces may remain available for some time at Dunn Loring, they said.