An estimated 100,000 people gathered on the National Mall in Washington, D.C. Sunday, to celebrate Earth Day, an event promoting protection of the environment.  

Wisconsin Senator Gaylord Nelson came up with the idea for the first Earth Day in 1970. 

This year's event, co-hosted by Earth Day Network and Green Apple Network, featured an eclectic lineup of musicians, celebrities, activists and politicians. 

In addition to the Washington, D.C. festivities, sponsors held coordinated volunteer events in Atlanta, Austin, Boston, Chicago, Denver, Los Angeles, New York City, San Francisco, and Seattle. Green Apple Network founder and event Executive Producer Peter Shapiro says he was inspired by President Barack Obama's call to community service during his inaugural address. 

In keeping with the theme, promoters emphasized "green" travel and offered a free bicycle valet service. Some concert-goers biked to the event, but most attendees chose to travel by Metro, the nation's capital's subway system. 

Click here to view Earth Day Slideshow - Photos by Daniela Schrier