AUG 31: Mr Gorbachev, who took power in 1985, opened up the then-USSR to the world and introduced a set of reforms at home.
But he was unable to prevent the slow collapse of the Soviet Union, from which modern Russia emerged.
Tributes have been paid worldwide, with UN chief Antonio Guterres saying he "changed the course of history".
"Mikhail Gorbachev was a one-of-a kind statesman," UN Secretary General Mr Guterres wrote in a Twitter tribute. "The world has lost a towering global leader, committed multilateralist, and tireless advocate for peace."
The hospital in Moscow where he died said he had been suffering from a long and serious illness.
In recent years his health has been in decline and he had been in and out of hospital. In June, international media reported that he had been admitted after suffering from a kidney ailment, though his cause of death has not been announced.
Russian President Vladimir Putin has expressed his deepest condolences following Mr Gorbachev's death, his spokesman Dmitry Peskov told Russian news agency Interfax, according to Reuters.
US President Joe Biden called him a "rare leader" and praised Mr Gorbachev as a unique politician who had the "imagination to see that a different future was possible" amid the tensions of the Cold War.
European Union President Ursula von der Leyen praised him as a "trusted and respected leader" who "opened the way for a free Europe".
"This legacy is one we will not forget," she added.
UK Prime Minister Boris Johnson said he admired Mr Gorbachev's courage and integrity, adding: "In a time of Putin's aggression in Ukraine, his tireless commitment to opening up Soviet society remains an example to us all."
Mr Gorbachev became general secretary of the Soviet Communist Party, and de facto leader of the country, in 1985.
At the time, he was 54 - the youngest member of the ruling council known as the Politburo, and was seen as a breath of fresh air after several ageing leaders. His predecessor, Konstantin Chernenko, had died aged 73 after just over a year in office.
Few leaders have had such a profound effect on the global order, but Mr Gorbachev didn't come to power seeking to end the Soviet grip over eastern Europe. Rather, he hoped to revitalise its society.
The Soviet economy had been struggling for years to keep up with the US and his policy of Perestroika sought to introduce some market-like reforms to the state run system.
Internationally he reached arms control deals with the US, refused to intervene when eastern European nations rose up against their Communist rulers and ended the bloody Soviet war in Afghanistan that had raged since 1979.
Meanwhile, his policy of glasnost, or openness, allowed people to criticise the government in a way which had been previously unthinkable.
But it also unleashed nationalist sentiments in many regions of the country which eventually undermined the stability of the country and led to its collapse.
In 1991, after a shambolically organised coup by communist hardliners failed, Mr Gorbachev agreed to dissolve the Soviet Union and left office.
He is seen in the West as an architect of reform who created the conditions for the end of the Cold War in 1991 - a time of deep tensions between the Soviet Union and Western nations, including the US and Britain.
He was awarded the Nobel Peace Prize in 1990 "for the leading role he played in the radical changes in East-West relations".
But in the new Russia that emerged after 1991 he has been on the fringes of politics, focusing on educational and humanitarian projects.
Mr Gorbachev made one ill-fated attempt to return to political life in 1996, receiving just 0.5% of the vote in presidential elections.
His divisive legacy is reflected in the tributes that followed his death.
Henry Kissinger, who served as US Secretary of State under President Richard Nixon, told the BBC's Newsnight programme that Mr Gorbachev will be "remembered in history as a man who started historic transformations that were to the benefit of mankind and to the Russian people".
James Baker, who negotiated the reunification of Germany with Mr Gorbachev's government, told the New York Times that "history will remember Mikhail Gorbachev as a giant who steered his great nation towards democracy".
But many Russians never forgave him for the turmoil that followed the collapse of the USSR.
Vladimir Rogov, a Russian-appointed official in occupied Ukraine, said Mr Gorbachev had "deliberately led the (Soviet) Union to its demise" and called him a traitor.
He will be buried in Moscow's Novodevichy cemetery, the resting place of many prominent Russians, next to his wife Raisa who died of leukaemia in 1999, Tass news agency said.
MAY 6: About 200 civilians are also believed to be hiding in bunkers in the plant.
UN chief Antonio Guterres said everything should be done to "get people out of these hellscapes".
Russia's Vladimir Putin said his forces are ready to give civilians safe passage, but fighters must surrender.
The Russian president has already declared victory in Mariupol, ordering his forces to seal off the sprawling industrial site - which was designed during the Cold War to serve as a nuclear bunker and has a network of tunnels deep below ground - rather than try to take control of it.
However the remaining Ukrainian forces inside the steelworks - fighters from the Azov regiment, some marines, border guards and police - say Russian attacks are continuing.
The Azov regiment released drone footage showing explosions at the steelworks, but the date of the footage could not be verified. Earlier on Thursday Sviatoslav Palamar, an Azov regiment commander, said Ukrainian defenders were fighting "difficult, bloody battles" with Russian troops who had managed to enter part of the complex.
But the Kremlin denied its forces had tried to storm the plant and insisted that a humanitarian corridor was open as part of a three-day ceasefire that started on Thursday.
Mr Guterres said a third operation to evacuate civilians from the city was under way and would arrive in the city on Friday. Ukrainian Deputy Prime Minister Iryna Vereshchuk said on social media that this would happen at about midday.
At least 20 children are thought to be among the remaining civilians in the steelworks, and food and water supplies are dwindling fast.
On Sunday an initial group of civilians was evacuated from the plant. Footage showed mainly women and children being helped to walk over piles of rubble, and boarding a bus with missing windows.
Russia launched its invasion of Ukraine 10 weeks ago but has not yet gained full control over any of Ukraine's major cities.
APRIL 29: Antonio Guterres said the council had failed to prevent or end the war in Ukraine.
This was "a source of great disappointment, frustration and anger," he said.
"Let me be very clear: [it] failed to do everything in its power to prevent and end this war," he added.
The 15-member UN Security Council is specifically tasked with ensuring global peace and security.
But it has faced criticism, including from Ukraine's government, for failing to act since the invasion began in February.
Russia is one of five permanent members of the body and it has vetoed more than one resolution on the conflict.
Mr Guterres was speaking on Thursday evening at a joint news conference with Ukrainian President Volodymyr Zelensky, who has previously criticised the Security Council.
"I am here to say to you Mr President, and to the people of Ukraine, we will not give up," he said.
But Mr Guterres also defended his organisation, admitting that while the Security Council had been "paralysed" the UN was taking other actions.
"The UN is the 1,400 staff members in Ukraine who are working to provide assistance, food, cash [and] other forms of support," he told the BBC.
At Thursday's briefing, President Zelensky said Mr Guterres had a chance to witness first-had "all the war crimes" committed by Russia in Ukraine.
The Ukrainian leader again described Russia's actions in his country as "genocide".
During the UN chief's visit, two blasts hit the central Shevchenko district of Kyiv, with three people taken to hospital with injuries, according to the city's mayor.
Mr Guterres also visited several sites where Ukraine accuses Russia of committing war crimes. Moscow denies the accusation.
In the town of Borodyanka, north-west of Kyiv, Mr Guterres spoke to reporters in front of buildings that had been destroyed by strikes and shelling.
He said the site made him imagine what it would be like for his own family, calling the war in Ukraine an "absurdity in the 21st Century."
And Mr Guterres made a passionate plea to save thousands of people in Ukraine's southern city of Mariupol, which has been virtually destroyed by weeks of heavy Russian bombardment.
"Mariupol is a crisis within a crisis," he said. "Thousands of civilians need life-saving assistance, many are elderly and in need of medical care, or have limited mobility. They need an escape route out of the apocalypse."
Russia has so far denied repeated requests by Kyiv to allow the last Ukrainian defenders and civilians trapped in the Azovstal industrial area to be evacuated.
But Mr Guterres later told the BBC that Russia's President Vladimir Putin had agreed "in principle" to allow civilians to evacuate the city.
Previous evacuation attempts have stalled and local officials have blamed Russian shelling.
NEW YORK, April 23: UN Secretary-General Antonio Guterres is set to visit Moscow to meet with Russian President Vladimir Putin and Foreign Minister Sergey Lavrov.
