CITY COPS last night found naked and barely clothed women, some participating in sex acts, inside a popular Mummers club in South Philadelphia, police said. Eleven people were arrested on prostitution charges.

The raid at the Downtowners Fancy Brigade clubhouse, at 2nd Street and Snyder Avenue, began about 7:30 p.m. and police were still on the scene after midnight.

Deputy Police Commissioner William Blackburn said that 10 women were charged with solicitation for prostitution, one man was charged with promoting prostitution and two other men were charged with liquor violations.

Following up on a anonymous tip six weeks ago, two undercover vice cops entered the club about 7:30 p.m. and were approached by several women between the ages of 20 and 47, Blackburn said.

About 50 patrons were inside and some of the women were naked and barely clothed and were engaged in sex acts, Blackburn told the Daily News at the scene. By 9 p.m. a search warrant had been obtained and the club was shut down, he said.

The tipster told police that women were solicited for sex on the second floor of the building every second Tuesday of the month between 7 p.m. and 11 p.m., Blackburn said. The undercover officers who were inside were approached by at least 10 women within an hour, Blackburn said, adding that the number would likely have been higher had police waited any longer to execute the search warrant.

The Downtowners is among the most successful Mummers brigades. According to its website, the group was formed in 1959, when it was accepted into the Fancy Brigade Division of the Mummers Parade.