Russia on Tuesday marked a half century since Yuri Gagarin became the first man in space, the greatest victory of Soviet science which expanded human horizons and still remembered by Russians as their finest hour.

As Russian state television proudly broadcast archive footage of the smiling Gagarin touring the world after his exploit, President Dmitry Medvedev described the flight as a "revolutionary" event that changed the world.

At 0907 Moscow time on April 12, 1961 Gagarin uttered the famous words "Let's go" as the Vostok rocket, with him squeezed into a tiny capsule at the top, blasted off from the south of the Soviet republic of Kazakhstan.

After a voyage lasting just 108 minutes that granted the 27-year-old carpenter's son historical immortality, Gagarin ejected from his capsule and parachuted down into a field in the Saratov region of central Russia.

From that moment on, his life, and the course of modern space exploration, would never be the same again.

"I believe it was a truly revolutionary event," Medvedev said in an interview with Chinese television, a transcript of which was published by the Kremlin.

"It was an outstanding achievement of Soviet space exploration, which divided the world into the time before and the time after the flight, which became the space era," Medvedev said.

The Soviet Union scored its greatest propaganda victory over the United States, spurring its Cold War foe to eventually retake the lead in the space race by putting men on the moon in 1969.

Russia's modern day rulers are using the anniversary to remind Russians of its past achievements and Medvedev was later to visit mission control outside Moscow and talk with modern astronauts on the International Space Station.

Later in the day, he is due to give a keynote speech on space exploration in the Kremlin that is expected to give an impulse for the future of the Russian space programme 50 years on.

In contrast to the tense battle of the 1960s, space is increasingly a matter of international cooperation with the ISS a joint effort between Russia, the United States and other partners.

Although Russia will this year take full responsibility for ferrying astronauts to the ISS when the shuttles are retired, its space programme has seen its share of problems in the run-up to the anniversary.

Three navigation satellites crashed into the ocean after launch, the latest launch for the ISS was delayed by a week due to a technical problem and the government has already said that space agency chief Anatoly Perminov is on the way out.

Medvedev said that Russia's space exploration had changed its focus from the first romantic days of space travel and now focused on practical results of its missions.

"There used to be a lot of idealistic vision regarding this. Now our efforts in space research have become more pragmatic," he said, while adding Russia still dreamed of inter-planetary and even inter-stellar exploration.

As well as the heroism of Gagarin, Russia is also remembering the genius of the man who created the rocket that put him into space and masterminded the flight -- chief Soviet rocket engineer Sergei Korolev.

One of the most remarkable figures in the history of space travel, Korolev survived being sent to the Gulag under Stalin to become a figure of such importance his role was only disclosed after his death in 1966.

The anniversary has also prompted a release of information from Russia about the most mysterious aspects of Gagarin's life, most notably his still unsolved death in a plane crash in 1968.

Declassified documents released last week said his jet likely manoeuvred sharply to avoid a weather balloon, prompting it to crash in a region outside Moscow and killing Gagarin and instructor Vladimir Seryogin.

Former Soviet space workers have also recalled how all those involved in the flight were forced by the Soviet Union to lie for years that he had landed in his capsule and not by parachute as was really the case.

This was because they feared the orbit would not be counted as valid by the international organisation monitoring spaceflight.