Kazakhstan's President Nursultan Nazarbayev, who has led the country since independence from the Soviet Union, has announced his resignation.

In a pre-recorded television address, he said the decision had "not been simple".

Mr Nazarbayev, 78, has been largely unchallenged since he became president of the oil-rich nation in 1990.

He has focused on economic reform while resisting moves to democratise the political system.

"I have decided to give up my powers as president," he said during his television address.

Born in 1940, Mr Nazarbayev came to power as first secretary of the Communist Party of Kazakhstan when it was a Soviet republic.

After independence, he was re-elected against largely token opponents in 1999, 2005, 2011 and - most recently - in 2015.

But the conduct of every election was criticised by foreign observers.

A huge country the size of Western Europe, Kazakhstan has vast mineral resources and enormous economic potential.

Since independence following the collapse of the Soviet Union in 1991, major investment in the oil sector has brought rapid economic growth, and eased some of the stark disparities in wealth of the 1990s.

BBC.COM