Fairfax County police launched an internal investigation yesterday into allegations of police brutality in a melee Saturday night at a Rte. 1 night club.

Six persons were arrested several patrons were struck with nightsticks and two of those arrested were treated at Mount Vernon Hospital after the disturbance, which occured about 10 p.m. in the One South Restaurant Night Club, 6320 Richmond Hwy.

Although no one as yet made a formal complaint alleging police brutality, several customers who were in the night club at the time say police confronted, assaulted and arrested people who were not interfering with the police.

A police spokesman said the internal investigation was requested by Deputy Chief Kenneth Wilson because the police report on the incident contradicts accounts given by witnesses.

Police said the disturbance started when officers Ernest L. Jones and William Petracca, of the Groveton district station, attempted to arrest Robert L. Lentz, 27, of 9207 Ox Road, Lorton, a temporary employe at the club who works there in the daytime as a dish-washers.

Police said that when they attempted to arrest Lentz he became violent and "began hitting and kicking the officers." Police said Petracca was struck in the back with a beer bottle during the fracas with Lentz.

Several witnesses said they saw no one hit the policeman with a bottle. Bill Howell, 23, who was near the police officers in the club, said the saw one policeman on top of Lentz while another hit Lentz with a night stick.

After Lentz was taken outside, police said, the two officers went back into the club to get two people who had interfered with their arrest.

Witnesses in the club, who said they saw no one interfere with the arrest of Lentz, said the officers returned and grabbed patrons who looked both surprised by and afraid of the policeman.

Howell, a regular customer at the night club and a salesman for Capital Lighting and Supply in Alexandrai, said the atmosphere in the bar became one of "absolute terror. People were afraid to move, to say anything."

Howell said he saw one of the policeman drag Lance Bergeron, 20, of 7135 Richmond Hwy, out of the club backwards by his hair.

Rebecca Wallace, 22, said she saw a dark-haired policeman return to the club and "grab a guy out of the corner, take him to the middle of the floor and beat him with his billy club."

Howell said the policeman appeared to "go absolutely berserk."

Police said yesterday they could not comment on the incident because of the internal investigation. On Sunday, Sgt. Ralph Benton, supervisor of officers Jones and Petracca, said he thought his officers did not use "excessive forc at any time and, as a matter of fact, I think they would have been justified if they had."

Police Lt. Jackie Watts, who arrived at the club with several other policemen after the arrest of Lentz, said Sunday he saw no excessive force used. He said police are authorized to "use any force necessary" in making an arrest.

Others arrested Saturday include Steven M. Demar, 20, 121 East Taylor Run Pky; Peter Leon Davis, 33, 5605 Elanor Ct.; William C. Banks, 23, 7212 Richmond Hwy., and Donald Ragen, 20, 6313 S. King Hwy., all of Alexandria.