VIRGINIA BEACH, SEPT. 3 -- Four people were injured and 100 businesses along a main thoroughfare in this resort city were damaged or looted early today when a riot erupted in a crowd of nearly 100,000 young people marking the end of summer. During the five-hour disturbance, which came on the second day of a fraternity and sorority festival commonly called Greekfest, two men were shot in separate incidents and a man was hurt in a fall from the fourth floor of a hotel. Another man was injured in an auto accident during the disturbance, police said. It was not clear tonight who shot the two men. Officials said police had not fired their weapons during the disturbance. More trouble broke out shortly after 8 tonight. A city spokeswoman said sporadic shots were fired and rocks and bricks were thrown at police after officials closed the city's beach area to incoming traffic. Helmeted state and city police responded by repeatedly marching through streets in the oceanfront area, using nightsticks to clear them as a police helicopter flew overhead. Mounted police and police dogs also were used in the sweeps. There were no reports of injuries or arrests in the latest violence. Officials said the area closed to vehicles stretched 10 blocks west from Atlantic Avenue and ran for more than three miles. At a news conference tonight Virginia Beach Mayor Meyera E. Oberndorf said the city was considering imposing a curfew Monday. "Virginia Beach will not stand by and watch as unruly indviduals trash our city and threaten people's lives and property," she said. Gov. Gerald L. Baliles authorized the use of the National Guard at 4 a.m. after local and state police were unable to restore order. City Manager Aubrey V. Watts Jr. asked state officials to suspend sale of alcoholic beverages until Tuesday, but a ban had not been announced as of 11 tonight. A spokesman said this afternoon that police had charged 555 people with various offenses, including 160 people who were arrested and taken to a precinct station. Tonight Oberndorf refused to say how many more arrests had been made or what those arrested were being charged with. John Thomas Jr., 28, of Forestville, was listed in serious but stable condition late today at Sentara-Norfolk General Hospital with a gunshot wound to the chest. The other wounded man, Charles Robinson, 25, of Roselle, N.J., was in fair condition at Virginia Beach General Hospital. Rodney Perry, 20, of Troy, Mich., was listed in critical condition at Norfolk General Hospital after being injured in the fall. Earnest Graham, 25, of Neptune, N.J., was in serious condition at Sentara-Norfolk General after an auto accident during the disturbance. The cause of the early morning confrontation between police and young people remained in dispute. City officials said that police moved in to disperse the crowd after looting started at shops and stores along the beach-front strip. However, several witnesses interviewed today said the riot began after police tried to force the crowd off the streets. Several students, many of them from black fraternities and sororities at schools along the Eastern Seaboard, complained of being harassed by police before the disturbance. At a news conference tonight, Virginia NAACP President Jack Gravely criticized the city's handling of the festival. Gravely said students had complained to him about the treatment they had received from police over the weekend. Although he said he was not able to substantiate the accounts, he said he had been told by students that they had been ticketed for cursing, jaywalking and riding shirtless in cars. After the initial disturbance, business owners took inventory at looted stores and boarded up broken windows. "Everything is smashed," said Jeff Willett, owner of Smith Brothers jewelry. "I have no insurance for anything like this." Broken glass was strewn about the store and six glass cases were empty. "Eighty-five percent of my goods are gone," Willett said. "I'm just numb." David Thomas, a 26-year-old employee of the Seaside Raw Bar, said he saw a young man pick up a trash can shortly after 2 a.m. and throw it through the window of a pharmacy. "Thousands of people were everywhere," Thomas said. "People were busting windows and stealing everything. I've never seen anything like this." Tourists who watched the events unfold said the streets turned into bedlam. "It was like police were trying to confine us," said Errin Smith, 20, of New York, who wore T-shirt beseeching, "Judge me not by the color of my skin, but by the content of my character." Smith said: "When they tried to move people, it went crazy. People were yelling 'Fight the Power.' At first people were just having a good time, but they harassed us from the minute we got here." Parker Rogers, who owns three gift shops along Atlantic Avenue, said that regardless of what police did to the young people, there "was no excuse for them to steal." "This is a very volatile situation down here," Rogers said. "Walking down the street, you hear people talking about taking over the city." Oberndorf said she was saddened by the oceanfront disturbance. "Virginia Beach is typically a family resort, and this sort of occurrence is highly unusual," she said. "I can assure you that this type of behavior is not condoned and will not be tolerated now or in the future." Greekfest began in 1985 as a relatively small beach party made up of predominantly black sororities and fraternities from southeastern Virginia universities, as well as Howard University. Last year, the number of students, which included some not in fraternities or sororities, swelled to 40,000. City officials said students blocked traffic, damaged hotel rooms and broke into some businesses. Last week there was growing anxiety about the coming Labor Day weekend, typically one of the busiest for this tourist city of 390,000, whose black population was estimated to be 10 percent in 1980. The city announced that beefed-up police patrols would be on duty and that the state police and National Guard had been asked to assist. Black leaders accused city officials of overreacting and said the city should have scheduled events for the students. "It makes the students feel awfully bad, unwanted," Gerald Tyler, director of university relations at Norfolk State University, said in an earlier interview. "We feel that people of color are not welcome at the beach. It has left many of the students with a bad taste in their mouth." City officials said the event was difficult to plan for because there was no one person or group to deal with. But Gravely said tonight that he and other NAACP officials had volunteered last week to serve as monitors during the event to defuse possible tensions, but that the offer had been rejected by the city. Today many of the students said it was unfair that many of them would be judged for the actions of a few. "You can't tell who did this," said Corrine Lochan, a senior at Brooklyn College. "The Greeks cannot take responsibility for the actions of others." Meanwhile, John Thomas Sr., father of the wounded Forestville man, said his son was shot while jogging on the beach. Thomas's girlfriend said they had just arrived in Virginia Beach about 2 this morning when Thomas, an avid runner, decided to go for a jog on the beach. About 3:30 a.m. he was shot in the chest. He does not know where the shot came from, she said, and there was no crowd around.