Crime in suburbia decreased on both sides of the Potomac in the first three months of this year as much as 28.5 percent below the same period last year, according to police statistics.

In Fairfax County, the number of serious crimes fell 28.5 percent from 7,049 in the first quarter of 1981 to 5,058 during the first three months of this year, said Police Chief Carroll D. Buracker. The "serious offense" category covers murder, rape, aggravated assault, robbery, burglary, larceny and motor vehicle theft.

"Except for a 1 percent increase last November, our serious crime figures have been dropping since last July," said Prince George's County Police Chief John E. McHale. "I can't remember a time like this when crime was down throughout the area."

Spokesmen from the Police Foundation and the Police Executive Research Forum, two Washington-based think tanks, said it is too early to say whether a long-range drop in serious crimes is indicated.

The suburban police chiefs offered a variety of explanations for the decline in serious offenses:

* The "blessing" of a winter in which temperatures repeatedly plunged to the single digits. "When it's extremely cold, even the bad guys don't want to go out," said Montgomery police Lt. James Lee.

* The introduction of precious metal ordinances throughout the area that require dealers to identify the sellers and supply records of purchases to local police departments.

* A dramatic decline in the market value of gold from a peak of $850 in 1980 to $337 currently and of silver from $50 to about $7.

* An increased awareness on the part of citizens who have formed hundreds of "neighborhood watch" programs in the area and have increased home security measures.

* A greater emphasis by police on undercover work and stakeouts designed to stop crimes in progress in high-crime areas.

* An increased emphasis on career criminals who commit multiple felonies.

In Montgomery County, major crime decreased 12 percent, despite a 52.9 percent rise (from 119 to 182) in aggravated assaults. Police Chief Bernard Crooke attributed that increase to the "frustrations of the times."

Crooke said burglaries continue to be the county's biggest crime problem, even though they decreased by 27.7 percent, from 1,390 to 1,005 in the first three months of 1982.

"Our arrest rates for burglaries reached an all-time high last year," said Crooke, "and there is a more serious interest within the communities. People know that it could happen to them."

Montgomery police also cited the work of their Special Assignment Teams, plainclothes officers who watch for suspicious activities in high-crime areas. From March through December of 1981, SAT teams arrested 992 suspects on more than 1,400 charges, said police.

The greatest decrease has occurred in Fairfax County, where 183 neighborhood watch programs are in operation. First quarter statistics show that residential burglaries in Fairfax plummeted from 1,353 last year to 641 in this year, a decrease of more than 50 percent.

Chief Buracker said Fairfax has a 16-member plainclothes unit and uses computers to identify "incipient crime patterns. These 16 officers target areas where we think we can apprehend offenders in the process of committing a crime."

Reports of serious crimes in Prince George's County fell by 8.8 percent, including a 14.3 percent decrease in robberies. Chief McHale said that stakeout teams are part of the reason, but added that citizens are not as careless as they used to be.

"FBI and Census Bureau statistics used to say that 20 to 50 percent of the burglars entered through unlocked doors and windows. People are taking better precautions," McHale said.

In Alexandria, which registered a 7.7 percent decrease in major crimes, police Cpl. Ralph Carlton agreed.

"We're doing about four to five home security surveys a day," said Carlton. "More of our police calls deal with attempts rather than actual break-ins now." The number of burglaries in Alexandria dropped from 623 in the first quarter of 1981 to 485 this year, said statistics.

In Arlington County, serious offenses dropped from 2,324 in the first three months of 1981 to 1,792 this year. Deputy Chief William Packett attributes the decrease partly to his department's habitual criminal units that work closely with prosecutors to seek higher bails and longer sentences for career criminals, but added this caveat:

"We don't take credit when crime goes up, so we really don't take it when crime goes down," said Packett.