Washington, April 22 : More than a billion people around the world celebrated Earth Day Tuesday with the theme 'Green Cities' calling for transformation in the public transportation systems to make them more accessible, convenient, and clean fuel-run.

The celebrations of the 44th edition of Earth Day also called for switching over to electric, hybrid, and other eco-friendly vehicles, the Earth Day Network reported.

Earth Day was first mooted by US Senator Gaylord Nelson in 1970 after an oil spill incident in Santa Barbara, California.

Twenty million people across the US rallied then for the protection of the environment.

By the end of 1970, the US Environmental Protection Agency was established, and efforts to improve air and water quality gained political traction.

Earth Day is now coordinated globally by the Earth Day Network, and is celebrated in more than 192 countries each year.

The Earth Day Network’s India Programme has its headquarters in West Bengal state's capital Kolkata.

The network in India organises workshops to provide environmental education and training for sustainable livelihood development among the youth.

The Earth Day Network also works very closely with the municipal corporations of major metropolitan cities in India to ensure that public parks and gardens are well maintained.

The network, in partnership with Rotary International, has launched 'Green Magic' which aims to plant 10 million trees across India. (IANS)