Countries around the world marked the 40th anniversary of Earth Day Thursday with a renewed call to combat global warming and protect the environment.
The first Earth Day event took place in the United States on April 22, 1970 and 20 million people participated in demonstrations to demand that the government take action to combat pollution.
Forty years later, the annual observance has become a global event, with an estimated one billion people in 191 countries taking part in a multitude of activities. Many events are coordinated by the U.S.-based non-profit group, Earth Day Network.
U.S. President Barack Obama marked the occasion with an event in the Rose Garden Thursday afternoon. Earlier, he called for continued hard work to "make the dream of a clean energy economy a reality" and pass on a safer and cleaner world for future generations.
A week of events in the U.S. began April 17 with environmentalists turning the National Mall in the heart of Washington into an eco-village.
U.S. officials are participating in education programs and community service projects over the next few days.
On the eve of Earth Day, Secretary of State Hillary Clinton said the world has come a long way in addressing environmental challenges. She said the U.S. is working more aggressively to reduce greenhouse gas emissions blamed for global warming.
U.S. Vice President Joe Biden announced the 25 communities that were selected to share more than $450 million in stimulus funding to make their homes and businesses more energy efficient.