The tall, thin man stood up on his bicycle pedals early yesterday and cranked down a gear as he pedaled up the slight incline along the 1500 block of New York Avenue NE. Tied to the back of the 10-speed white bicycle were two travel bags and a large wrapped package.

A short distance away, Jim Nelles was driving a nondescript dark red car and, unbeknown to the bicyclist, keeping a trained eye on him. With one hand, Nelles steered his car along the service road and with the other he held a two-way radio close to his mouth.

"Get ready to grab him," he barked. "He's heading east and he's in front of Hecht's now. Okay, get him!"

While early morning rush-hour commuters stared from their cars, two blue-and-white District of Columbia police cruisers instantly followed Nelles' command, swooped in on the wrong side of the road and neatly trapped the man on the bicycle between them.

Nelles, a six-year police veteran who prides himself for thefts from cars, had engineered another one. Within two minutes, Nelles and other officiers had recovered the two travel bags and unwrapped the packages, finding a television set and a plastic bag full of cash. All of it, the police believed, had just been stolen.

The arrest of the bicyclist, Carl E. Martin, 25, on a grand larceny charge, was part of a police response to an overall 20 percent increase in the number of reported crimes in the city during the last six months. Thefts of valuables from cars represent the largest portion of that increase, according to police officials.

But for Nelles and his partner, Steve Wyland, Martin's arrest represented the chief product of their work Monday night, a nine-hour stakeout of the New York Avenue motel parking lots where thefts from cars are common. o

The seven motels strung along New York Avenue near the District-Maryland boundary are popular tourist spots. Although motel managers have posted signs warning guests against leaving valuables in cars, many unsuspecting tourists do just that. Police say the people who lose cameras, citizen's band radios and other easily fenced items, often express surprise that such incidents would occur at large, well-lighted motels in the nation's capital. But they do.

"Larceny alone has pushed [the number of crimes up in the area surrounding New York Avenue] from 18 to 27 per day," said Deputy Chief Carl Profater, commander of the Fifth Districk. "All we can do is keep after it."

One of the methods of keeping after it is the "distance surveillance" which Nelles believes he has perfected over the past five summers, the peak season for thefts from autos.

Larceny from cars is one of the most difficult crimes to stop, police say, because of the difficulty of catching a thief in the act of stealing. Large hotel parking lots and dark streets give the thief a good chance to break into a car quickly and be gone in less than a minute, making routine police scout car patrols ineffective.

Using the stakeout method, Nelles said, he makes one to three arrests a night.

"Distance surveillance" is the basic stakeout of TV and movie fame, except that it's done from a rooftop instead of a car.

For Monday night's surveillance, Nelles and Wyland arrived at work dressed in jeans, T-shirts and running shoes. They loaded one lawn chair, two sleeping bags, four blankets, two shirts, two heavy coats, two flashlights, one pair of binoculars and the usual police equipment in Wyland's truck.

The sleeping bags, they noted, were for warmth, not for sleeping.

While Wyland unpacked the cold weather gear on the moonscape-like roof of a building overlooking the 1600 block of New York Avenue, Nelles explained the procedure.

"One of us watches the parking lots and the other checks the street," he said. "You've got to watch every minute because these guys are fast."

Below the officers' well-hidden roof-top perch, tractor trailers roared by, teen-agers drag-raced each other west toward Bladensburg Road, and tourists slowed to inspect the hotels for vacancy signs.

Wyland gazed at the full moon and wondered aloud if it or the near-perfect weather would increase the night's criminial activity. Nelles, his muscular body crunched down in the yellow and white lawn chair, stared across the road at a hotel parking lot.

"I feel it in my bones," he said. "This is going to be a good night." And then he added a prediction based on five years of watching for car thefts -- and his bones. "It will be at one o'clock," he said.

At 11:30 p.m., Nelles observed two teen-agers breaking into a parked school bus and called in patrol cars to check it out. The youngsters were charged with attempted unauthorized use of a vehicle.

"That puts the meat on the table," observed Nelles. "It makes the watch worthwhile."

But it was clear that Nelles wouldn't be satisfied with the menu until he got what he had come for: larcenies.

At 1 a.m., a determined Nelles settled deeper into the chair and announced that there was still time to catch a thief. But nothing happened.

At 2 o'clock, he and Wyland changed positions. And every two hours after that, the men alternated chair-sitting and roof-sitting as they watched the now quiet avenue below them. Still nothing.

Nelles and Wyland continued to watch the well-lit stage of street and parking lot till dawn. But no actors showed up to play the role of thieves.

A downcast Wyland, on his sixth night of working with Nelles, checked his watch and announced that it was time to give up.

"It's such fun when you catch them," said Wyland. "You watch them actually do it and then they deny it. It's like catching someone with their hand in the cookie jar."

Nelles continued to sit in the lawn-chair and flick pebbles at the rats scurrying around a trash pile down below.

"If I come out here and sit, I want to catch someone," he said. "If you go to the crime areas, the crime will come to you."

Reluctantly, the two men gathered up their gear and climbed down from the roof. Wyland went ahead to the police station while Nelles made a check on a car that had looked suspicious when it arrived during the night.

Stoped at a street light, Nelles talked about the possibilities for the next night. "We'll do better tonight," he said. He glanced over at the parking lot of the Travel Lodge and cursed. "Look at that guy there on the bicycle," he exclaimed. "He's up to no good."

A man sitting astride a bicycle appeared to be squeezed between a rental truck and a car. He fiddled with a pack on his bike. As Nelles made a U-turn, his body straightened up. With 10 minutes left on his shift, he grabbed for his radio and called for other cruisers heading into the station to help him out.

Five minutes later he had made the long-awaited arrest.

Nelles rode the suspect's bike back to the Travel Lodge while the suspect was given a free lift -- in the back of a squad car. Nelles, the now excited larceny expert, found a door open on the truck and the front seat littered with belongings.

The truck's driver, Robert Ellis, was awakened in his motel room.

Ellis, a freshman at Georgetown University Law Center, had arrived from Detroit three days earlier. He identified the television, the bags and the money as his.

As he watched an officer check his Tv for fingerprints, he shook his head in wonderment.

"I've been staying with friends in Southeast for the past three days. That TV was right on the front seat. When I ran out of couches to sleep on, I came to the hotel," he said. "I thought everything would be safe here on New York Avenue.'