Performances from headliners Lorde and Muse were already underway when the evacuation was ordered.
Organizers said they decided to cut the performances short due to an approaching storm and warnings from the National Weather Service.
"We are disappointed to have to end today's performances early, however our first priority is the safety of our fans, staff and artists," said Sandee Fenton, Director of Publicity for C3 Presents, the promoter behind Lollapalooza.
If there is severe weather or another emergency situation at Lollapalooza, there are shelter sites in underground parking garages around Grant Park.
One of the shelters is at Grant Park North, located at 25 North Michigan Avenue between East Madison and East Washington streets. Another shelter is at Grant Park South, located at 325 South Michigan Avenue between East Jackson Boulevard and East Van Buren. Another shelter is at the Millennium Lakeside garage located at 5 South Columbus Drive, between East Monroe and East Randolph streets.
There are signs to direct people to these locations. Safety officials will be available to give directions.
More than 170 performers will hit eight stages at this year's four-day music fest. Wiz Kalifa, Lorde, Muse and Cage the Elephant were a few of Thursday's headliners. CLICK HERE for the full concert schedule.
Gates opened at 11 a.m. Festival-goers will need wristbands to get in and will be subject to a security screening. Large bags are prohibited and all bags will be searched. Attendees can re-enter up to three times per day.
CLICK HERE for additional information on allowed and prohibited items, as well as tips for attending Lollapalooza, click here.
About 100,000 people are expected to attend Lollapalooza each day. Although general admission packages quickly sold out, there is an official ticket exchange where people can buy and sell tickets at lollapalooza.com.
"We have a plan of a schedule that we want to go see. But, you know, it falls to pieces and you really see whoever sounds good," said Blake Shreves, who arrived early for the music festival.
"The storms? Actually, I look forward to the storms. All the dirt on the inside turns into a mudslide. It's one big party. Love it," said Gavin Humfleet, who also arrived early for Lolla.
Parking will be available, but public transportation is probably the best to get to Grant Park. Lollapalooza organizers arranged shuttles from several different suburban locations. Divvy will have special valet service at the Jackson and Michigan station for bike riders.