Russian President Vladimir Putin has paid tribute to Mikhail Gorbachev following his death, saying he had a "huge impact on the course of world history".
The last leader of the Soviet Union was someone who found his "own solutions to urgent problems", Putin said.
Mr Gorbachev died on Tuesday aged 91, it has been announced.
He died after a long illness, undergoing treatment at the Central Clinical Hospital in Moscow in recent weeks.
Dictator Putin said in a condolence telegram sent today that he knew that huge changes were desperately needed.
Political heavyweight Gorbachev ended the Cold War but failed to prevent the collapse of the USSR.
Putin said he "deeply understood that reforms were necessary" and strove to offer his own solutions to the problems faced by the Soviet Union in the 1980s.
Expressing his condolences to his family, the Russian president said in a statement: "He led our country during a period of complex, dramatic changes, large-scale foreign policy, economic and social challenges."
Putin also credited the "great humanitarian, charitable, education activities" carried out by Mr Gorbachev.
The men haven't always seen eye to eye, with the former Soviet leader becoming a strong critic of the leader in recent years.
He said Putin's inner circle was full of "thieves and corrupt officials" and demanded the leader change his style and "not be afraid of his own people".
A Kremlin spokesman then described Mr Gorbachev as a "unique person... who will always remain in the history of our country".
It is unclear how Gorbachev died, but in June we reported that he was taken to the hospital suffering from a serious kidney ailment.
Prime Minister Boris Johnson has said that he is "saddened" to hear that Mikhail Gorbachev has died in a "time of Putin's aggression in Ukraine", while Joe Biden paid tribute to him as a "man of remarkable vision".
Tributes to Gorbachev also came from the world of showbiz, with Arnold Schwarzenegger hailing the twentieth century leader as "one of my heroes" and saying he was "unbelievably lucky to call him a friend".
"All of us can learn from his fantastic life", he added.
