MOSCOW --- Mikhail Gorbachev, who changed the course of history by triggering the demise of the Soviet Union and was one of the great figures of the 20th century, has died in Moscow aged 91.
His death was announced on Tuesday by Russian news agencies, which said Gorbachev had died at a central hospital in Moscow "after a serious and long illness".
Gorbachev, in power between 1985 and 1991, helped bring U.S.-Soviet relations out of a deep freeze and was the last surviving Cold War leader.
His life was one of the most influential of his times, and his reforms as Soviet leader transformed his country and allowed Eastern Europe to free itself from Soviet rule.
The changes he set in motion saw him lionised in the West -- he won the Nobel Peace Prize in 1990 -- but also earned him the scorn of many Russians who lamented the end of their country's role as a global superpower.
He spent much of the past two decades on the political periphery, intermittently calling for the Kremlin and the White House to mend ties as tensions soared to Cold War levels after Russia annexed Crimea in 2014 and launched an offensive in Ukraine earlier this year.
His relationship with President Vladimir Putin was difficult at times, but the Russian leader nonetheless expressed his "deep sympathies" after Gorbachev's death.
"In the morning, (Putin) will send a telegram of condolences to his family and friends," Kremlin spokesman Dmitry Peskov told Russian news agencies.
Gorbachev spent the twilight years of his life in and out of hospital with increasingly fragile health and observed self-quarantine during the pandemic as a precaution against the coronavirus.
Gorbachev was regarded fondly in the West, where he was affectionately referred to as Gorby and best known for defusing US-Soviet nuclear tensions in the 1980s as well as bringing Eastern Europe out from behind the Iron Curtain.
He won the Nobel Peace Prize for negotiating a historic nuclear arms pact with US leader Ronald Reagan, and his decision to withhold the Soviet army when the Berlin Wall fell a year earlier was seen as key to preserving Cold War peace.
He was also championed in the West for spearheading reforms to achieve transparency and greater public discussion that hastened the breakup of the Soviet empire.
In a statement, U.S. President Joe Biden credited Gorbachev with having "the imagination to see that a different future was possible and the courage to risk his entire career to achieve it".
"The result was a safer world and greater freedom for millions of people," he added.
British Prime Minister Boris Johnson, meanwhile, said he "always admired the courage and integrity" Gorbachev showed in bringing the Cold War to a peaceful conclusion.
"In a time of Putin's aggression in Ukraine, his tireless commitment to opening up Soviet society remains an example to us all," he said in a Twitter post.
UN chief Antonio Guterres praised Gorbachev as "a one-of-a-kind statesman who changed the course of history" and "did more than any other individual to bring about the peaceful end of the Cold War".
'Man of peace'
French President Emmanuel Macron praised him as a "man of peace whose choices opened up a path of liberty for Russians. His commitment to peace in Europe changed our shared history."
The first Russian leader to live past the age of 90, he was congratulated by world leaders, including U.S. President Joe Biden and former German chancellor Angela Merkel on his 90th birthday.
At home, Gorbachev remained a controversial figure and had a difficult relationship with Putin.
For Putin and many Russians, the breakup of the Soviet Union was a tragedy, bringing with it a decade of mass poverty and a weakening of Russia's stature on the global stage.
Many Russians still look back fondly on the Soviet period, and Putin leans on its achievements to buttress Russia's claim to greatness and his own prestige.
As the USSR collapsed, Gorbachev was superseded by the younger Boris Yeltsin, who became post-Soviet Russia's first president.
From then on, Gorbachev was relegated to the sidelines, devoting himself to educational and humanitarian projects.
He made a disastrous attempt to return to politics and ran for president in 1996 but received just 0.5% of the vote.
Over the years, he saw many of his major achievements rolled back by Putin.
Supporter of free press
An early supporter of Russia's leading independent newspaper Novaya Gazeta, founded in 1993, he donated part of his Nobel winnings to help it buy its first computers.
But the newspaper, like Russian independent media across the board, came under increasing pressure during Putin's two-decade reign.
Novaya Gazeta, whose chief editor Dmitry Muratov last year won the Nobel Peace Prize, suspended publication in late March after Moscow's military intervention in Ukraine.
Gorbachev himself made no public statements about Russia's military action in Ukraine, though his foundation called for "an early cessation (to) hostilities and immediate start of peace negotiations".
Uniquely among Soviet leaders, Gorbachev made no secret of his warm and supportive relationship with his wife Raisa, an elegant woman who often appeared in public with him and whose premature death from cancer was a devastating blow.
A source close to the Gorbachev family told news agency TASS that he would be buried next to Raisa at Moscow's Novodevichy cemetery, the resting place of many other famous Russian figures, including Yeltsin.