Montgomery County has surpassed Baltimore and Prince George's as Maryland's population leader while the District's population has dropped below 600,000, according to the preliminary 1990 Census count released yesterday.

The city's census coordinator said the District would challenge the preliminary count, as it did successfully a decade ago when the Census Bureau said its population had fallen below 600,000 for the first time since 1930.

Fairfax County is the most populous jurisdiction in the Washington area and in the state of Virginia, with 815,223 people, the preliminary figures show.

The total population of the Washington area climbed to 3,694,038, an 18.9 percent increase since 1980.

Overall, Virginia's population went up 14.6 percent in 1990, to 6,127,680. Maryland gained 12.2 percent more people between 1980 and 1990, raising its population to 4,732,934.

Except for the sharp decline in the District's population, most of the counties, cities and towns in the Washington area experienced double digit growth, the preliminary count indicated.

Outside the Capital Beltway, the growth in some suburbs during the 1980s was faster than in some Third World countries. The population of Bangladesh, for example, grew by one-third during the last decade, while Loudoun and Prince William counties in Virginia grew by 49 percent and the population in Howard County went up 57 percent.

Fairfax County accounted for more than a quarter of Virginia's new population during the 1980s. Four Maryland counties in the Washington area -- Anne Arundel, Howard, Montgomery and Prince George's -- accounted for two-thirds of Maryland's growth during the decade.

"This shows that the spurt of growth we had in the late 1980s as higher than anticipated," said Jay Langford, chief of planning analysis for the Metropolitan Washington Council of Governments. "It means the birth rate and migration is higher than anticipated."

The census figures are eagerly anticipated by local officials throughout the region because they are used to calculate allocations of federal aid and political representation; the more people, the more money and political clout. They also are widely used for local planning decisions, and many businesses refer to census data in deciding on marketing and location.

Conflicting with the District's own population forecasts, census officials said Washington's population is 574,844, down 10 percent from the 1980 count of 638,432. Officials had projected that the city's population would fall to 628,300.

Not since the 1930 census, when the District counted 487,000 people, has the city's population been below 600,000. In 1980, the city disputed the initial count, which was 568,300 people.

The District's census coordinator, Kay Hixson, said yesterday that even before the numbers were released, the city was preparing to contest the preliminary count, which she said would cost the District hundreds of millions of dollars in federal aid if allowed to stand.

"Our indications are that we may have lost some {people}, but nowhere near that level," said Hixson, adding that the city would issue a statement on the census Monday.

The District was not the only government to challenge the preliminary numbers a decade ago. After the 1980 preliminary numbers were released, census officials raised the final tallies for Anne Arundel, Montgomery and several other area jurisdictions.

Census officials acknowledged that the numbers released yesterday are incomplete. By design, the Census Bureau issues a preliminary count in the summer for local officials to review and challenge within 15 days.

In most cases, a census spokesman said, the final count is higher than the first one, which means most of yesterday's preliminary figures are likely to be increased.

District officials believe the low count stems from several factors, including a protest by local homeless people who refused to be counted, obstacles to counting foreign-language-speaking people, including illegal immigrants, and an increasing number of people living in group homes not counted by the census.

Stuart Bendelow, chairman of a COG regional forecasting committee, said District officials always have argued that the city's population has stabilized because of the influx of foreigners and others, but in reality "no one had a good handle on the District's population."

Montgomery's new status as Maryland's most populous jurisdiction also was a mild surprise. Forecasters had said the county's population would rise to 730,000 people this year, which was 20,816 fewer than the initial census count of 750,816 people.

Montgomery easily surpassed Baltimore, which now has 720,100 people, and Prince George's, with a population of 719,812.

County Executive Sidney Kramer said his staff had expected Montgomery to overtake Baltimore within the next two years, so "this comes a little earlier than expected."

Although Montgomery's new status as Maryland's largest jurisdiction means the county is vibrant, Kramer said, it also means "we must now provide services for that influx of population."

Forecasters also were slightly off on the Fairfax count, to the county's favor. They estimated 810,263 people in 1990, compared to 595,754 people in 1980. The preliminary count was 815,223, up about 37 percent in a decade.

"Assuming the numbers are accurate and the formulas don't change, there's an opportunity for us to receive additional revenues," County Executive J. Hamilton Lambert said yesterday.

The figures also confirmed that Virginia Beach has surpassed Norfolk as the state's most populous city. Virginia Beach's population rose 48 percent, to 388,737 in 1990.

Jurisdiction......1990 (preliminary)..1980 (final)......% Change

Maryland Total.......4,732,934.........4,216,933......12.2%

Anne Arundel...........423,941...........370,775......14.3

Baltimore County.......687,918...........655,615.......4.9

Calvert.................50,674............34,638......46.3

Charles................100,478............72,751......38.1

Frederick..............149,550...........114,792......30.3

Howard.................186,131...........118,572......57.0

Montgomery.............750,816...........579,053......29.7

Prince George's........719,812...........665,071.......8.2

St. Mary's..............75,062............59,895......25.3

Baltimore City.........720,100...........786,741......-8.5

District of Columbia...574,844...........638,432.....-10.0

Virginia Total.......6,127,680.........5,346,797......14.6

Fairfax County.........815,223...........595,754......36.8

Arlington..............170,089...........152,599......11.5

Loudoun.................85,760............57,427......49.3

Prince William.........215,862...........144,636......49.2

Stafford................59,232............40,470......46.4

Spotsylvania............56,942............31,995......78.0

Cities and Towns

Alexandria.............111,620...........103,217.......8.1

Fairfax City............19,740............20,537......-3.9

Falls Church.............9,538.............9,515.......0.2

Norfolk................229,718...........266,979.....-14.0

Manassas................28,039............15,505......80.8

Manassas Park............6,564.............6,524.......0.6

Richmond...............201,166...........219,214......-8.2

Virginia Beach.........388,737...........262,199......48.3

SOURCE: U.S. Census Bureau