President Vladimir Putin said on Sunday constitutional amendments approved in a nationwide vote created the conditions for Russia's "progressive development" for decades to come.

One of the changes approved in the week-long vote that ended on July 1 makes it possible for Putin to seek two more terms as president and, if re-elected, to stay in power until 2036.

Other changes grant former presidents immunity from prosecution, enshrine a reference in the constitution to God, offer pensions protection and define marriage as a union of a man and a woman.

Opponents said the vote was illegal and illegitimate, and an independent monitoring group said the vote was deeply flawed.

But Russian news agencies quoted Putin as saying: "I am absolutely convinced that we are doing the right thing in adopting amendments to the current constitution."

"They will strengthen our nationhood and create conditions for the progressive development of our country for decades to come," he said.

The Kremlin has hailed the overwhelming vote in favor of the amendments as a triumph for Putin, who said on Friday that Russian society had shown unity by voting to back the reforms.