Russia is celebrating the 50th anniversary of the first human space flight when cosmonaut Yuri Gagarin completed a single orbit of Earth.It will be marked by ceremonies and a 50-gun salute at the Kremlin in Moscow.
Russian President Dmitry Medvedev said it was a "revolutionary" event that changed the world.
Gagarin's achievement earned him instant global stardom, and dispelled fears humans could not survive beyond the Earth's atmosphere.
Since his flight in 1961, more than 500 men and women have followed in his footsteps.
Mr Medvedev is to visit mission control outside Moscow and talk with astronauts on the International Space Station to mark the event.
Later in the day, he will deliver a keynote speech on the future of the Russian space programme.
'Let's go' Before Gagarin, no-one knew for sure if a human could withstand the conditions in space, says the BBC's Steve Rosenberg in Moscow; some believed weightlessness would induce madness, that the G-forces on take off and re-entry would crush the body, and there was concern over the effects of radiation.
But when Gagarin's face and voice were beamed down from space, the world saw that the cosmos was not to be feared - it was to be explored, our correspondent says.
On 12 April 1961, to the cry of "Let's go!", Yuri Gagarin embarked on a voyage lasting 108 minutes in a tiny two-metre-wide (6ft) capsule, then ejected and parachuted down into a field in central Russia.
"The most emotional moment was when we heard he was walking and waving; his arms and legs were whole. We understood in one sigh that our five to six years of hard work had paid off and we had achieved something huge," said veteran cosmonaut Georgy Grechko, now 79, who worked as an engineer on Gagarin's space capsule.
The US responded 10 months later, when John Glenn made the first US orbital flight.
Unlike in Gagarin's time, space is no longer the preserve of two superpowers, our correspondent adds.
Today as well as Russia and America, there are other players in space, including Europe, China and India - with their own programmes and their own vision for space exploration, he says.