Fairfax County police yesterday dropped prosecution of a drug charge against an Annandale youth who claimed he was arrested improperly during a search of his apartment aimed at its previous occupant.

Police denied they decided against prosecuting because of the claim by the defendant, Christopher V. Hall, 18. Both police and officials of the Commonwealth's Attorney's office said they declined to prosecute because he would have been classified as a first offender. Hall had been charged with possession of one ounce of marijuana, an offense punishable by up a year in jail plus a $1,000 fine, although first offenders usually are given probation.

Hall's lawyer John Zwerling of Alexandria, stood by the claim that Hall was arrested improperly following the drug raid on Hall's apartment Dec. 14. He criticized police for detaining Hall and his roommate for nearly three hours at a police station on charges of marijuana possession, a misdemeanor for which they could simply have been issued a summons.

"They endangered these youngsters by making them walk back home 2 1/2 miles at 3 o'clock in the morning," Zwerling said. "They got carried away with this thing."

The grounds for the search warrant for the raid were based on statements by officers who had gone to the apartment a week earlier to investigate a burglary and allegedly noticed evidence of marijuana. The occupant at that time vacated the apartment the next day, shortly before Hall moved in, Zwerling said.

Zwerling challenged the warrant's legality, but General District Court Judge G. William Hammer never ruled on the motion because the prosecution was dropped. Zwerling criticized police for seeking the warrant, however, saying such action would discourage victims of crimes from seeking police assistance.

Commonwealth's Attorney Robert F. Horan Jr. disagreed sharply. "People who are going to report crimes are going to go on reporting crimes," he said. "I don't think the police can make a policy of ignoring certain misdemenors. If people think a law shouldn't be on the books they should go to the legislature and get it changed."