WATCH: ABC7's Richard Reeve reports on the heat the Girl Scouts had to cope with on the National Mall today.

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WASHINGTON (AP) - About 200,000 Girl Scouts are gathering on the National Mall in Washington to celebrate the organization's 100th anniversary with celebrities including Mandy Moore.

Organizers say the Saturday event will be the largest gathering of Girl Scouts in history with girls coming from around the world. They hope to set a Guinness World Record. Girls from every state are expected to attend.

Moore, a former Girl Scout, will address the crowd. Events are planned around the Washington Monument from 9 a.m. to 4 p.m.

The "Girl Scouts Rock the Mall" will feature performers including Erin Willett, a finalist on TV's "The Voice" and "American Idol" finalist Thia Megia.

Other activities include yoga, a fashion show and a news quiz game show from the Newseum, a museum about journalism.