The Scouts and supporters came together Thursday on the National Mall for the event, dubbed "Girl Scouts Rock the Mall," The Washington Post reported.
Organizers said they were expecting 200,000 people to show up for the celebration.
"It's symbolic of the important place Girl Scouting has had in this country," said Anna Maria Chavez, chief executive of Girl Scouts of the USA.
The day's events were mainly musical performances, which included local groups, as well as American Idol's Thia Megia.
Unfortunately, with temperatures soaring into the 90s, 63 people were treated for heat-related incidents, with 20 of them having to be transported to the hospital, CNN reported.
First aid tents saw a steady stream of people all day who were complaining about the heat, Deputy Chief Kevin Byrne said. Most people returned to the festivities after having some ice or water.