CHICAGO -- The second day of Lollapalooza came to a sudden halt around 3:30 CT on Saturday when threatening weather forced organizers to evacuate the venue. Loudspeaker announcements from the stages informed the 60,000-plus concertgoers in Chicago's Grant Park that they had to immediately make their way to the exits due to the threat of high winds, hail, lightning and a potential tornado.

The unprecedented move for the massive lakeside gathering came almost a year after a stage collapse caused by high winds killed seven people at a Sugarland concert at the Indiana State Fair.

"Our first priority is always the safety of our fans, staff and artists," said Shelby Meade, communications director for C3 Presents, the promoter behind Lollapalooza in a statement. "We regret having to suspend any show, but safety always comes first."

While many fans heeded the request to leave, nearly 30 minutes after the first warning thousands remained milling around. A spontaneous mosh pit popped up at the corner of Columbus and Balbo, headed by shirtless young men jumping up and down while shouting 2 Live Crew lyrics.

Shortly after 4 p.m. amid shouts from defiant patrons who refused to leave citing the nearly $100 they had paid for a day of music, a cordon of police and security agents began advancing on the stragglers.

Troy Whittle, 19, a native of nearby Hoffman Estates, Illinois, was attending his sixth Chicago Lollapalooza and as he walked toward the crowded exit onto Michigan Avenue he expressed disappointment that the show was being postponed. "There was no way I thought they were going to do this," said Whittle, who admitted he snuck into the concert with several friends. "There was rain last year during Deadmau5 that was crazy and they didn't do this."

As the sky turned black and thunder clapped, it caused those still stubbornly hanging around to give a yelp, but a look back provided evidence that the park appeared to have been emptied calmly and without incident.

One hour after the evacuation call was made, it proved to be the right one, as the sky turned black, lightning flashed, sheets of sideways rain poured down and concert attendees who had nowhere to go to escape the deluge huddled under hotel awnings and inside Starbuck's coffee shops.

With the fast-moving storm expected to pass within an hour, it was still possible that organizers would finish out the day's music, which was to include sets by the Red Hot Chili Peppers, Frank Ocean and Santigold, with an amended lineup.

Lollapalooza officials said they were coordinating with the Chicago Office of Emergency Management and Communications (OEMC) to monitor weather conditions and determine when the park is safe for festival-goers to return.

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