A 20-year-old Wheaton man was arrested last night and charged with felony murder in the death of a young woman found strangled inside the Old Post Office Pavilion after the boisterous New Year's Eve celebration there, D.C. police reported.

The victim was identified yesterday as Marquita Vaughn, a rebellious, attractive Dunbar High School senior who lived at 259 V St. NW. She turned 18 on Sunday, and the city-sponsored New Year's Eve celebration in downtown Washington was a symbol of her new-found freedom, her mother said. When she left the family house to join friends that night, she did not say goodbye.

Police identified the suspect as Alex Tyrone Simmons, of 11212 Markwood Place, a waiter at a restaurant inside the pavilion. According to police, the victim and the suspect were acquaintances.

In explaining the charge of felony murder, Capt. Larry Soulsby, head of the police homicide unit, said the victim was raped before being killed.

Vaughn's partially clad body was discovered shortly after 2 a.m. on New Year's Day in a remote, southwest area of the Pavilion's ground floor, behind a stairway, according to police. The D.C. medical examiner's office said that a preliminary autopsy showed she died of strangulation. Tests also are being conducted to determine whether she had been drinking or had used drugs.

"Turning 18, she saw it as a statement of being legally grown," said Patricia Wright, the victim's mother. "Nobody would be able to tell her what to do any more. She said that all the time. Now my baby's dead."

The stairs near where Vaughn's body was found were open to the public and were used regularly by revelers as an alternative to the packed elevators.

Soulsby said that the victim and the suspect "were acquaintances" who met through another acquaintance "and had known each other for some time."

He said she had apparently gone to the area under the stairwell "of her own volition."

Law enforcement sources said a witness told of seeing Vaughn earlier in the evening with two men. Efforts by police to find the two led them to Simmons, and all three men were interviewed, the sources said.

Police said Simmons was arrested about 10 o'clock last night at police headquarters. According to police, Simmons was a waiter at the Blossoms restaurant in the Pavilion.

Friends and family described Vaughn as a bright, sweet young woman who was interested in men, but was naive and easily manipulated.

Vaughn told family members she was bored with school and showed more interest in her classes at a beauty school. She told relatives she wanted to work as a secretary for the federal government.

But several months ago, she stopped going to school and became a discipline problem for her mother, according to family members. Vaughn moved out of her mother's house, in the 700 block of Kennedy Street NW, and moved in first with her boyfriend and later with her grandmother on V Street NW.

Her mother cried yesterday as she recalled her estrangement from her daughter and her efforts to persuade her to pay more attention to school and less to her social life.

"She was so confused," Wright said. "The girl was seeking help. I tried to get her help [from social service agencies], but nobody would listen. She didn't listen."

Vaughn's body was found by a cleanup crew in a cramped cubbyhole under a metal staircase near the Pavilion's rear entrance on 12th Street NW. The building is located on Pennsylvania Avenue between 11th and 12th streets NW. The stairs are at the opposite end of the building from the indoor stage, where bandleader Cab Calloway and other musicians entertained about 4,000 people with tickets to get inside the Pavilion.

Police estimated that 100,000 persons attended the indoor and outdoor festivities around the Pavilion, but an official in the mayor's office who was involved in planning the activities said yesterday that only about 30,000 attended.

Besides the death, there were numerous incidents of fighting and bottle-throwing outside the Pavilion as rock bands played into the night and early morning. Beer, wine and liquor were sold and there was widespread drinking, according to police, some of whom questioned the wisdom of allowing alcohol sales at the event. Some groups of teen-agers wandering through the packed crowds grabbed purses, and others linked arms and pushed forward pell-mell, causing fights.

The violence was the worst in three years of New Year's celebrations at the renovated landmark on Pennsylvania Avenue. Mayor Marion Barry, who has eagerly promoted the affair, dubbed this year as "Putting on the Glitz in 1986," to compete with New York's annual Times Square party.

Police officials said that 313 District police officers were on duty inside and outside the Pavilion, an increase of 38 over last year. In addition, the Post Office Pavilion Joint Venture, the company that operates the shopping mall, hired additional security guards for the event, according to Fani Brown-miller, the Pavilion's general manager.

Police said that despite sporadic incidents, the event generally was orderly given the crowd's size. There were 15 arrests made in and around the building, 12 of which were for disorderly conduct.

"It was a madhouse," said one police officer, who attended the celebration off duty, who did not want to be identified. "If I had to give it a rating, I'd give it a 'C' . . . . There was a lot of sporadic fighting, but it was pretty much quelled by the citizens themselves . . . . There was a lot of brotherly love, and people saying, 'Hey, let's stop all this fighting.' "

Barry said that he is "very concerned" about the death and other incidents and has ordered a "complete report" from police about the events. A spokeswoman for the mayor said yesterday that he would not comment on the security procedures until he has seen the report.

Members of Vaughn's family said that they did not report the young woman missing on New Year's Day because she had failed to return home before. But family members became concerned when they saw television news reports that described the victim found at the Pavilion as being short, dark complexioned and wearing a pantsuit, white blouse and gray ankle-high boots.

Wright said she called her mother, Ethel Brown, who was the last family member to see Vaughn alive, and asked what Vaughn had been wearing when she left the house. The grandmother described the pantsuit, and Wright said she immediately became frightened.

"Mama, that sounds like 'Quita,' " Wright recalled saying to her mother.

Detectives came to her house with photographs of the dead woman, and family members identified her.Staff writers Linda Wheeler and Lyle V. Harris contributed to this report