Russia on Sunday marked the 50th anniversary of the world's most horrific but long-classified space catastrophe when 126 people were burned alive during a launch pad accident.

During the accident, which the Russian space agency says was a veritable "inferno", people were burned alive or vaporised altogether, while others died of noxious fumes or succumbed to burns later.

Authorities and relatives of those who died in this accident and others held a memorial service at the Baikonur cosmodrome and also laid flowers at their mass graves.

In 1960 the Soviet Union, locked in a space race with the United States, was developing an intercontinental ballistic missile known as the R-16, and on October 24 that year was scheduled to launch a prototype rocket when it exploded on the launch pad.

"People died in horrific pain, essentially burning alive, but the country and the rest of the world practically never learnt anything about that terrible catastrophe and its heroes-victims," Russian space agency Roscosmos said.

"To this day it is considered the most horrific (tragedy) in the history of space exploration," the agency said in a statement.

The Russian space agency, citing Soviet scientist Boris Chertok, says 126 people died, but also notes that the exact number of casualties is hard to pin down and may range between 60 and 150.

The testing crew accidentally initiated the second stage of the rocket, which ignited the first stage causing the disaster.

Those closest to the rocket were "more or less vaporised, and many of the victims only later succumbed to their burns," the space agency said in a separate statement in English.

Known as the "devil's venom", the rocket fuel was so noxious that those who jumped into blast wells to escape the "inferno" were found asphyxiated, the space agency said.

In the West, the tragedy is referred to as the Nedelin disaster, after the commander of the Russian Strategic Missile Forces, Mitrofan Nedelin, who oversaw the rocket programme and died along with designers and testers.

During the test, Konstantin Gerchik, head of the Baikonur cosmodrome at the time, asked Nedelin to step aside for safety reasons.

Nedelin refused. "Am I not the officer just like the rest?" Gerchik remembered Nedelin telling him, according to excerpts of his memoirs carried by the state news agency RIA Novosti.

The only thing that was left of the marshal was a pin by which he was later identified, according to Gerchik's previously classified memoirs.

Soviet authorities led by Nikita Khrushchev imposed total secrecy over the accident.

The Pravda newspaper, the official mouthpiece of the Communist Party, said Nedelin had perished in a plane crash, NTV channel said, which estimated 74 people burned alive and more than 50 received injuries.

The files on the launch failure were only declassified in the 1990s.

By coincidence, on the same day three years later a fire at a launch pad killed another seven testers.

In the wake of the two accidents, October 24 is known as "a black day" for space exploration on which Russian officials commemorate the memory of all those who dedicated their lives to the space programme.

Space officials do not schedule any launches on this day.

Sending the first man into space in 1961 and launching the first sputnik satellite four years earlier are among key accomplishments of the Soviet space programme and remain a major source of national pride in Russia.