A 12-year-old boy was arrested in Southeast Washington Monday for carrying a 9mm semiautomatic handgun, touching off a new round of concern by police about the firepower on Washington streets.

Already this year, District police said, they have confiscated 178 weapons, most of them of the sophisticated automatic and semiautomatic genre.

Most of the weapons were seized during drug-related arrests, police said. Some had been stolen or used in the commission of other crimes.

Monday's arrest occurred after police were called to Third Street and Livingston Road SE by two school security guards who were tipped off that a boy was carrying a gun near Friendship Elementary School. When the security guards stopped the boy, police said, a handgun fell out of the waistband of his pants.

Six rounds were found inside the clip of the gun, one round in the chamber. The boy was walking around with the gun cocked, police said.

Police, citing the sensitivity of the case because of the youth's age, declined to say why he was carrying the gun or where he got it. Police said the gun was worth as much as $700 retail at a gun store and twice that much on the black market.

Semiautomatic weapons are "all over the place," one police officer said. "It's getting scary."

Police attribute the sophisticated weaponry now found in the District to the flourishing drug trade, which they say has added tremendously to the homicide rate. In the past 50 days there have been 51 homicides, police said, 21 this month.

Increasingly, officers are calling for the department to begin equipping them with 9mm handguns, said Detective Dave Israel, chief shop steward of the Fraternal Order of Police's 7th District station.

"There's a real heavy push" for the weapons, Israel said. The biggest threat police are facing is posed by weapons that often have two or three times the number of bullets as the .38-caliber service revolvers that officers carry, he said. Several officers across the country were killed last year while reloading their service revolvers during gun battles, he said.

Automatic weapons are those that continually shoot bullets as long as the trigger is squeezed, police said. With semiautomatics, the trigger must be released and squeezed before each shot, but the guns carry large numbers of bullets stored in clips that can be swiftly reloaded.

Most of the 9mm semiautomatics consfiscated, including that found on the 12-year-old, can hold 16 bullets at a time, police said, compared with six bullets for police service revolvers. However, many semiautomatic and automatic weapons are capable of holding even more ammunition.

Police recently confiscated a .22-caliber rifle, for example, that had a clip capable of holding 100 rounds of ammunition. Police also have seized such weapons as the Uzi, the M11 and the TEC9, all capable of holding 32 rounds of ammunition.

Although Monday's arrest shocked many officers, Israel said a Jan. 7 incident in which two officers were shot at better illustrates the dangers police face when confronting people with sophisticated arms.

After a Southeast car chase, one of three fleeing men opened fire on the officers. In seconds the assailant shot eight rounds at the officers -- three of which struck the police cruiser. That is two more rounds than either officer had in his service revolver, Israel said.

Gary Hankins, chairman of the Fraternal Order of Police, said after the shooting that he asked Police Chief Maurice T. Turner Jr. to equip patrol cars with shotgun racks as an interim step to equipping officers with 9mm weapons.

"In a pursuit, the shotgun should be in the front seat so that it's immediately available," he said. Current police policy allows officers to check out shotguns, he said, but it makes them store the weapons in the trunks of their cars.

Hankins said he has not received a reply from the chief.

Although the department has begun studying the use of 9mm hndguns by police, officials are several months from reaching any decision, said a police spokesman, Capt. William White III.

"We're outgunned. That's all there is to it," said Joseph Masson, a D.C. police firearms examiner.

Police records show that more weapons are seized each year. In 1984, the department seized 1,990 weapons. Last year, D.C. police confiscated 2,483 weapons.