About 70 Fairfax County police officers, some armed with automatic weapons and arrest warrants, descended on a Mount Vernon neighborhood early yesterday and arrested 20 persons on drug-related charges, police said.

In another early morning raid, this one in the District, city police charged six persons -- two of them juveniles -- and seized 106 "rocks" of the cocaine derivative crack valued at about $9,000. Also seized were drug paraphernalia and three weapons, including a machine gun and a semiautomatic .22 caliber handgun.

The Fairfax County arrests, most of which were made in apartments and condominiums, culminated a four-month undercover operation aimed at street-level drug dealing, police said. Two men were arrested at the county jail, where they were being held on other charges, police said, and about 10 warrants were outstanding last night.

David McKenzie, manager of the Janna Lee Apartments, where most of yesterday's arrests occurred, said he had sought help from county police because drug dealing in the southeastern Fairfax neighborhood was "getting out of hand." He said that he posted no-trespassing signs around the complex after he noticed an increasing number of District license tags, but that tenants still came to him daily asking, "Can't you do anything about this?"

Unknown to the residents, McKenzie said, two undercover police officers have been living in their midst in an apartment that he provided rent-free for the last four months. The officers bought small amounts of cocaine in the area, police said.

Lt. Col. John V. Rob, deputy police chief, said the neighborhood had been a problem area for street-level drug dealing for some time. He said he could recall only a few operations of this size in the county in the last five years.

The four-hour operation began at 1 a.m. as about 70 officers, most in marked cars, slipped into the neighborhood, single file. "They looked like an army," McKenzie said. "It went very smoothly."

Eight teams of officers, some from the tactical team, which uses automatic weapons, served arrest warrants to residents at Janna Lee Apartments and adjacent Colchester Towne condominiums, while other officers set up a roadblock at Tamarind Street and Janna Lee Avenue.

No drugs were seized, but one handgun was confiscated, officers said.

Most of those arrested were charged with distribution of cocaine, police said. In all, there were 44 charges, some for selling crack.

D.C. vice and SWAT team officers were also heavily armed yesterday when they pounded on the door of a second-floor apartment in Southeast.

In addition to the weapons seized in the 800 block of Chesapeake St. SE, police said they found toy guns.

In the apartment "was the typical scenario that New Yorkers come into D.C., they find somebody who is vulnerable, offer them money or drugs to use their apartment to sell cocaine and to administer cocaine as a crack house where you buy cocaine and smoke it," said Lt. Robert Sheaffer, commander of the 7th District vice squad, which conducted the raid with the assistance of the Emergency Response Team.

Four adults were charged in U.S. District Court yesterday with possession of crack with intent to distribute.

All four were ordered held without bond by U.S. Magistrate Jean F. Dwyer pending a preliminary hearing. During a brief hearing yesterday, Dwyer said that the case involved "three generations of a single family" and that this "upsets me more than a little bit."

A 14-year-old boy and a 17-year-old boy also were arrested, but their names were not released.

Dwyer refused a request from the apartment renter, identified as Theresa Foreman, to be released on bond, saying that if she had accepted money for the use of the apartment, "she is just as guilty as the rest of them."

The complaint stated that Foreman told police she was being paid $100 a month to permit her apartment to be used to sell crack.

When the officers arrived, one of the men charged, Kenroy R. Laing, 19, was standing in the doorway of the building and fled inside, stuffing something inside his pants, according to to the complaint filed in U.S. District Court.

Later, 89 bags of crack were allegedly recovered from Laing's trousers and 17 more bags from the dining room table, the complaint stated.

In addition to those charged, five people were served subpoenas to testify before a grand jury in the case.

Staff writer Nancy Lewis contributed to this report.