Activists and organizations around the world are marking the start of Earth Day Tuesday.

Earth Day is celebrated every year on April 22nd. Events are held worldwide to demonstrate support for environmental protection.

One focus of this year's Earth Day is the Green Cities Campaign, which calls for an investment in renewable energy.

Earth Day was first celebrated in 1970, and is now coordinated by the Earth Day Network.

More than 192 countries celebrate the holiday every year.