A warm July night, thousands of weekend revelers and a stepped-up police presence on South Street proved to be a recipe for a chaotic Saturday night that ended with a slew of arrests and five injured cops.

Ironically, the police mobilization may have been for an event that didn’t even take place — an annual Greek Picnic of African-American fraternities in Philadelphia that has been the scene of past rowdiness but was dramatically scaled back.

But Greek Picnic or not, Philadelphia’s bar-lined party street was mobbed with people on Saturday, and as the night dragged on there were a series of encounters between police and partiers.

A Daily News editor, Albert Stumm, happened on the scene and described mounted police marching six-abreast clearing out the streets.

He said cops on horses made him get out of a cab and forced him to run down the block as they threatened to arrest him after saying “What, are you dumb?" and “Don’t make me tell you again."

Lt. Frank Vanore, a police spokesman, said that state police joined with city cops in patrolling South Street, which had been the epicenter of unrest on prior Greek Picnic weekends and which had also been the scene of a violent “flash mob” of youths blamed for several beatings in May.

Vanore said that at the height of the unrest involving mostly 14-20 year olds, from 11:30 p.m. to 1:45 a.m., a part of South Street was closed to vehicle traffic.

Some 19 people were arrested during the operation, including eight for disorderly conduct, four for obstructing the highway, one on narcotics charges, one for retail theft, and five with assaulting a police officer.

Vanore said five officers were hurt, including one who was hit with a closed fist in front of 222 South St. and required stitches, while another officer was hit by a vehicle but not seriously injured. The spokesman said that a couple of cops were also kicked or hit during the episode.

Meanwhile, a video that was making the rounds on the popular Web site YouTube.com last night depicts a moment of chaos at the height of South Street unrest. Entitled “GREEK WEEK BEATDOWN 2009,” it shows two officers forcefully slamming an apparent suspect to the pavement of South Street.

Sharif Smith, 24, shot the video for his company, RGP. Though South Street was already packed with revelers, Smith said the rain did little to dampen the crowd’s enthusiasm. “Once it started drizzling, more people came out...” He said they crowded the street, and “kept trying to sit still,” sometimes in front of traffic. That’s when the struggle with the cops started.

“I was just there at the scene getting footage and it broke out in front of me,” he said. “It was a guy tussling with police — he got more and more hyperactive,” said Smith. “Then it was five of them just trying to subdue him.”

“There were lots and lots of people,” Joe DiStanislao, who was driving home south on 6th Street, said. “It created an illusion of chaos.”

Sgt. Ray Evers said that the troublemakers were apparently not affiliated with Greek Picnic revelers. “It’s not the college kids that cause the issues,” he said, “but kids from different neighborhoods who hear its the place to be, and spoil it for the other group.”

Andre Bright, president of the Pan-Hellenic Council said that Greek Picnic was cancelled and a smaller celebration, dubbed “Greek reunion” was held instead. It drew about 1,500 people, far lower than the tens of thousands it used to draw in the ‘90s, he said. The festivities were over by 6 p.m., he said, and the vast majority of Greek Reunion attendees went to parties at their respective fraternities or to a step-squad performance at the Blue Horizon.

“I would be surprised if any of the people who went to Greek [reunion] went to South Street,” he said.

Staff writer Regina Medina contributed to this article.