Environmental groups and governments around the world have observed Earth Day with events aimed at protecting nature and raising awareness about global warming.

In cities in the United States and Canada Sunday, citizens and politicians participated in tree-plantings and environmental cleanups.

The non-profit Earth Day Network said hundreds of millions of people were expected to take part in more than 170 countries.

The group says the annual holiday celebrates a milestone this year, because China's leadership has mandated that all local governments do something to mark Earth Day.

In the United States, environmental groups are urging people to take steps to decrease the amount of greenhouse gasses in the atmosphere - by driving less, driving more efficient vehicles and replacing conventional light-bulbs with energy-saving fluorescent bulbs.

At a White House Correspondents' dinner in Washington Saturday, singer Cheryl Crow and movie producer Laurie David got in a heated argument with President Bush's advisor Karl Rove. The pair said they urged Rove to take a new look at global warming, but Rove says they intended to insult him and they succeeded.

The first Earth Day was marked in the United States on April 22, 1970. Twenty million people took part in protests around the country that day to demand the government combat widespread pollution.

Some information for this report was provided by AP and Reuters.