Legendary Soviet-era cosmonaut Alexei Leonov, who in 1965 became the first person to venture out of a spacecraft in orbit, has died in Moscow aged 85 after a long illness.
Russian space agency Roscosmos made the announcement on its website on Friday, without providing a cause for his death. Russian media earlier reported that Leonov had had health issues for several years.
"His name will forever remain in history," Roscosmos said in a letter to Leonov's family.
"His greatest memorial will be our country's future achievements in space travel, to which he dedicated his heroic life."
NASA broke into its live televised coverage of a spacewalk by two Americans outside the International Space Station to report Leonov's death."A tribute to Leonov as today is a spacewalk," Mission Control in Houston said.
On March 18, 1965, Leonov made history when he left a spacecraft during the day-long Voskhod-2 (Sunrise-2) mission for a spacewalk that lasted 12 minutes and nine seconds.
This came as the Soviet Union and United States, Cold War foes, were locked in a frantic race to conquer space.
Leonov's spacewalk preceded the first US one, by Ed White, by less than three months.
On his second trip to space 10 years later, Leonov commanded the Soviet half of Apollo-Soyuz 19.
He was twice decorated with the country's top honour, the Hero of the Soviet Union.
'Loss for the whole planet'
Born in Siberia, Leonov was a fighter pilot before becoming a cosmonaut. He was a friend of Yuri Gagarin, who became the first man in space in 1961 and trained with him.
Tributes poured in from across the world on Friday, with Western space agencies and experts praising Leonov's contribution and admirers bidding him farewell.
Russian President Vladimir Putin sent his condolences to Leonov's loved ones, Kremlin spokesman Dmitry Peskov said.
"Putin always admired Leonov's courage and thought he was an extraordinary man," Peskov told Russian news agencies.
NASA on Friday offered sympathy to Leonov's family, saying it was saddened by his death.
"His venture into the vacuum of space began the history of extravehicular activity that makes today's Space Station maintenance possible," NASA said on Twitter.
British astronaut Tim Peake described the Russian as "a hero to so many" who was "always full of wisdom and humour".
"This is a loss for the whole planet," said Russian cosmonaut Oleg Kononenko.
Leonov's feats were immortalised in cinema and literature.
Stanley Kubrick's 1968 film 2001: A Space Odyssey included recordings of Leonov's breathing in space.
The late science fiction writer Arthur C. Clarke named a Soviet spaceship after him in his "2010" sequel to "2001: A Space Odyssey."
The 1965 mission was also celebrated in a 2017 Russian film, "The Spacewalker" where Leonov acted as consultant.
Leonov, who will be buried on Tuesday at a military memorial cemetery outside Moscow, is survived by his wife, a daughter and two grandchildren.