The patrician streets of Georgetown have seen a sharp increase in street crime like muggings and vandalism -- things that once seemed completely foreign to Washington's ritziest neighborhood.

The increase in crime follows last summer's appearance of large crowds of young people in Georgetown on weekend nights. Police were startled when up to 20,000 youths gathered on several balmy nights in January and February, raising the prospect of even larger crowds this summer.

There were 36 reported street robberies in Georgetown this January, more than double the 15 that were reported in January 1984. Robberies declined in February and March, as winter returned and police increased their street patrols, but authorities fear that crime will increase again as the weather gets warmer.

Last summer, as the crowds of youths arrived, little crime came with them, said Capt. Rod Murray, who heads the police department's Georgetown detail.

"Now we're getting the guys who grab pocketbooks, wallets and gold jewelry, and it's the girls' gold chains that are leading the hit parade," Murray said. "Street crime is starting to build in Georgetown."

The weekend crowds are swollen by 18-year-olds from Virginia and Maryland who can no longer drink legally on their home turf but are able to have beer and wine in the District -- and who choose to cruise Georgetown's many bars. One bar, Winston's, even has special college nights and helps pay for chartered buses that haul students from as far away as the University of Maryland at College Park.

As more young people congregate in Georgetown, more businesses decide to cater to them. And as the streets fill up, police say, it becomes easier for a robber to grab a necklace or a purse and escape into the crowd. It is a cycle that disturbs many longtime neighborhood business persons and residents, who fear that Georgetown's traditional upscale clientele is being frightened away.

"We've been in business for 22 years," said John Laytham, part-owner of Clyde's, an M Street bar and restaurant, "and now we have to pay two off-duty police officers each $15 an hour to keep our doorway clear. That has become the price of doing business in Georgetown . . . . I have people who come to lunch here who say they won't come out to Georgetown for dinner anymore."

Dave Roffman, editor of the Georgetowner newspaper and chairman of the crime prevention committee of the Citizens' Association of Georgetown, said he gives an update on crime in Georgetown at the monthly meeting of the citizens association.

"The changes I am seeing are the street robberies," said Roffman. "We are having purse-snatching right at Wisconsin and M, right out in the open. It is an anything-goes atmosphere."

Rhonda Epps, 21, was one of 10 persons robbed in Georgetown the night of Feb. 23. She had just bought some records and was walking back to her car when a man ran up and grabbed her purse.

"I grabbed it back and then he grabbed it," Epps said. "I swung my fist at him and yelled, 'Give it back,' but he took off running. I chased him for four blocks till he ran back to a car and I lost him."

Both Epps and her assailant are black -- raising a touchy issue, because the increase in young people coming to Georgetown has included an influx of blacks. But police say they arrest as many whites as blacks in Georgetown.

"Its the crowds that are scary," said Officer Mike Gallahan, a 12-year veteran. "They remind me of a bunch of frenzied sharks. The black kids understand when you tell them to move on, but the white kids will argue with you. They have liquor courage and they will cause a fight. Our arrests work out to about 50 percent black and 50 percent white."

Gallahan said that he believes that the hostility toward the police has escalated since last summer. "Ordinary people will try to punch a cop," he said. "It's because of the mobs. They go crazy. You're trying to make an arrest, and someone tries to get a punch in at you. They grab parts of your uniform like your handcuffs, hat or gun. They want to take home a souvenir of Georgetown."

Gallahan said he used to patrol with one other officer. "Now we go in groups of four or five so you'll have someone to watch your back."

Jim Rienzo, night manager at Winston's, said that on weekend nights the bar has seven doormen and an off-duty police officer to keep the peace. Most of the bar's customers are 18 to 22 years old, he said.

"This is a wild bar," said Rienzo, 25. "They dance on the bar and they throw up on themselves. This is definitely a young bar."

Rienzo said that he had noticed a marked increase in the number of young people coming from Virginia, which recently raised the drinking age. In Virginia, the age for drinking hard liquor is 21; for drinking beer and wine it is currently 19 and will rise each year until it reaches 21. The drinking age for all forms of liquor in Maryland is 21. In the District, the drinking age is 21 for hard liquor and 18 for beer and wine.

Rienzo said his bar sometimes contributes $75 to the cost of buses chartered by college students who want to come to special college nights at Winston's. "We have buses coming in from the University of Maryland and Catholic University," he said.

"It's getting so it feeds on itself," said Laytham, the part owner of Clyde's. "As this age group takes over, more and more businesses cater to them and that in turn attracts even more of them until the whole thing will just spin out of control."

Laytham's partner, Stuart Davidson, said he believes the crowds and the street crime have made the area "not user-friendly. I am taking precautions now at night," said Davidson. "For me it's not being here. It is only safe inside."

The citizens association, the local Advisory Neighborhood Commission and the Georgetown Business and Professional Association have all resolved to ask for better lighting on Georgetown streets and an increase in police patrols.

For years, residents of Georgetown have resisted more lighting on residential streets. The old-fashioned lampposts and the low light seemed in keeping with the vintage houses and brick sidewalks.

Plans call for keeping the same light fixtures but installing bulbs that are six times brighter than the current ones, and doubling the brightness of the lights on the commercial corridors.

"Our officers have a hard time seeing the length of the block with the lighting the way it is now," said Murray. "The citizen groups now feel this is a necessary change, and we hope to have the lights installed on several blocks in the next few weeks."

Murray said that he has asked for "substantial" increase in officers to patrol Georgetown on Friday and Saturday nights. Last summer, he had about 60 officers, both uniform and plainclothes, assigned to deal with the 10-block area of Georgetown.

Murray said a big police department van will be parked near Wisconsin and M streets starting in early April to process arrested people on the spot, thus freeing officers to return more quickly to patrol duty. Now they have to make a one-mile trip to the station house in upper Northwest.

Other plans, according to police officals, include a fleet of tow trucks to remove illegally parked cars and a parking ban on several blocks of Wisconsin and M Street. Police also say they will crack down on minor violations such as littering, obstructing the sidewalk and urinating in public.

On Friday night, police officers started writing $10 tickets for "walking against a red light."

Roffman said that the citizens association has two priorities for Georgetown.

"First, we have to get the crowds back in order," said Roffman. "The police will have a lot tougher attitute and the word will filter down to the kids that they can't do those things anymore."

"Second, we have to get the word out to the good people that it is safe to come back to Georgetown," said Roffman. "If they don't come back, all we will have is young kids on the streets. And the only businesses which will survive will be the junk places that sell ice cream and T-shirts."