District of Columbia police yesterday broadened their sweep of the city's drug-selling haunts, arresting marijuana buyers and sellers around Chapin Street NW while continuing a four-day-old crackdown on heroin sales in the Shaw neighborhood.

By last night, police said, they had arrested about 70 people on drug charges this week.

In a quick but unobtrusive operatio yesterday, detectives from the narcotics unit sat in unmarked cars along 14th Street and waited to nab marijuana buyers as their cars sped away from the drug scene in the 1400 block of Chapin.

"There's a BMW coming down," one undercover officer reported over a police radio after he witnessed a drug transaction.

Within seconds, a beige BMW rolled down oneway Chapin Street and turned left on 14th Street. Detectives in three unmarked cars roared off after it. Two blocks away they caught up with the car and surrounded it, bringing it to a halt.

A tall man in blue jeans, with a tennis hat pulled down around his ears, was ordered out of the car and told to stand spread-eagled against the car. He was frisked, handcuffed, placed in an unmarked car and taken to headquarters. The charge: possession of marijuana.

Minutes earlier, a van bearing Virginia tags had been stopped at the corner of 14th and W streets. The undercover lokout on Chapin Street said the occupants had just purchased marijuana.

As curious bystanders and startled 14th Street motorists looked on, half a dozen officers searched the three occupants and the van. Three men were arrested and charged with possession of marijuana.

"What you messing with me for?" asked another man as he got out of his white Toyota after being stopped by police. The police surveillance on Chapin Street had picked him out.

"I was just riding through town," said the man, dressed in a three-piece blue suit and tie.

"Yeah," a detective replied with a shrug. "Just shut up and get in the van."

From 2 until 3 p.m., detectives made 12 arrests of marijuana buyers and dealers on the bustling Chapin Street block, where a steady stream of youths run up to thw windows of slow-moving cars to sell marijuana in bags costing $5 and $10, known in street parlance as "nickel and dime bags."

One of the youths arrested was 13. Police said he was selling the marijuana for an adult. Sgt. Ramon Gonzalez, furious that a youngster was selling drugs, at one point walked up and down Chapin Street, telling the people siting on the sidwalks and stoops that he was looking for the "SOB" who would have a kid peddle drugs.

"If we donht get him today, we'll get him before this thing is over," said Capt. James P. Nestor, head of the narcotics unit.

Nestor said yesterday's operation in the Chapin Street area was in addition to the drug raids that have been occurring since Tuesday in the vicinity of 7th and T strets NW. Nestor said police will soon be raiding other drug-trafficking areas, including the Southeast Washington neighborhoods of Condon Terrace, 15th Street and Independence Avenue, and Talbert Street at Martin Luther King Avenue.

"The idea of this thing is to go everywhere," he said.

The police activity on Chapin Street was welcomed by some residents who had complained about the illegal trafficking for months. However, an ice-cream vendor said: "It ain't going to do no good. The only way they can stop selling on the street is to stop the imp;orters and ain't nobody going to do that."

Meanwhile, for the fourth day, uniformed officers and narcotics detectives broke up a teeming crowd of about 300 people in the Shaw drug-selling area, sending heroin dealers and addicts scurrying to nearby strets to complete their deals.

Officials said fewer arrests were being made yesterday afternoon because some officers now have to spend time in court on arrests made earlier in the week.

But a contingent of more than two dozen uniformed officers again spread out to surrounding streets, trailing the heroin dealers and breaking up knots of people that formed on side streets.

One of the most dramatic arrests yesterday was not drug-related, and came before the now-daily influx of officers. At about 2:30 p.m., D.C. police who had been trailing three men sought in a Prince George's County robbery earlier eysterday, blocked the suspects' car, briefly causing a massive traffic jam on heavily traveled 7th Street.

The high-profile uniformed officers conducting the anti-heroin drive had been kept out of the area for an extra 45 minutes yesterday afternoon while others officers moved into position to arrest the robbery suspects.