World leaders including Russian President Vladimir Putin, US President Joe Biden and German Chancellor Angela Merkel congratulated former Soviet leader Mikhail Gorbachev on his 90th birthday on Tuesday.
"You rightly belong to those bright, unconventional people, extraordinary statesmen of our time, who have had a significant impact on the course of national and international history," Putin wrote in a congratulatory letter to Gorbachev, published by the Kremlin.
Gorbachev's "great professional and life experience" still allowed him to "actively participate in popular social and educational work" as well as "international humanitarian projects," Putin wrote.
"Your commitment to freedom and your courage over the decades to make the tough, albeit necessary, decisions, have made the world a safer place," Biden wrote in a letter released by Gorbachev's staff.
The letter said that the agreed extension to the US-Russian nuclear arms treaty START was proof that the two countries would continue Gorbechev's "legacy."
German Chancellor Merkel joined Putin and Biden in congratulating Gorbachev, who was one of the architects behind German reunification more than 30 years ago.
"I take your day of honour as an opportunity to thank you once more for your personal commitment for the peaceful overcoming of the Cold War and the completion of German unity," Merkel wrote in a letter to the former leader.
"Your important contribution to a reunification in freedom remains as unforgotten in Germany as your constant personal engagement for friendly relations between our two countries," Merkel said.
Gorbachev, the former secretary-general of the Communist party, is considered one of the greatest reformers of the 20th century.
He introduced policies which opened up the Soviet Union to the world, and ultimately led to the collapse of the Communist regime and the end of the Cold War with the United States.
Many Russians also say he is responsible for the ensuing downfall of the Soviet Union in 1991.
Pro-democratic forces however see him as a symbol of freedom, as he has criticized repression under Putin and warned against falling back into a dictatorship. Gorbachev also owns a political foundation and co-owns the Kremlin-critical newspaper Novaya Gazeta.