When the refrigerator mechanic and young conscript soldier Anatoly Babi was given the chance in the autumn of 1968 by his military superiors to 'see the world', he leapt at the opportunity. The son of peasant parents born in the Soviet republic of Kirghizia, set off in an army lorry to Hungary, where he joined a large force of his fellow Soviet soldiers.

What Babi did not realise then was that he was part of a 100,000-strong force of troops from the Soviet-led Warsaw Pact that, exactly 40 years ago this week, were to roll into Czechoslovakia and crush the 'Prague Spring', the liberalisation movement led by the country's Communist party First Secretary, Alexander Dubcek. His attempt to introduce democratic reforms into the rigid socialist state and give socialism a 'human face' had captured the world's imagination. It led to six months of reform - from abolition of censorship to the recognition of human rights - an extraordinary period in Cold War history, which the Czech documentary film-maker and historian Jan Kaplan described to The Observer as 'a brief loosening of the straitjacket of party rule, a deep breath of fresh air before being submerged again'.

Babi, then 26, soon realised that his adventure was far more dangerous than his commanders had let on. The Russian-Ukrainian soldier was with the artillery that was to go into action if full-scale war broke out. 'We were given gas masks. We were told to sleep in our clothes. We were told to expect big exercises. But when we were given real bullets and set off in a convoy on rails, I understood that it was no game,' the now retired father of five told The Observer in an interview at his house in the country outside Moscow.

'The commanders said there had been a coup, an uprising, and it was our job to make sure there was no repeat of Hungary 1956,' he said, referring to the revolt against Hungary's Stalinist government. 'We were told we were fulfilling our international duty,' he said.

He recalled the reception they received from Czechoslovak citizens. 'They threw bricks and cobblestones at us and even tried to set fire to our vehicles,' he said. 'I understood then that they didn't see us as liberators.' The hatred culminated in injury for Babi when an angry old Czech man threw a grenade at him while he was trying to disarm a band of resistance fighters.

In the week when tanks have again been dispatched by Moscow to another country, Babi has allowed The Observer to publish his snapshots of that time, including a ghostly image of his comrades in a pine forest and one of himself standing next to his lorry. 'Because we lived under communism, we assumed the Czechs were happy with that system too,' he said. 'If I had known then what I know now, I would not have been so desperate to go ... but would have stayed at home and fixed fridges.'

The brothers Jan and Bohumil Hajny were among millions of Czech citizens who watched as the tanks rolled into the country and past the front door of their family house in the Prague district of Hostivar. Factory worker Jan, also 26 at the time, recalled his shock at realising that the six heady months of the Prague Spring - when Czechoslovakia flirted with the dream that the communist system could be turned into 'socialism with a human face' - were over.

'We received a phone call at 6am that the Russians had arrived at the airport and were taking it over,' Bohumil Hajny said. The first reaction of the amateur photographers was to go out and capture the events with their cameras, a Chinon semi-automatic and a Praktika, despite the protests of their concerned mother. The extraordinary shots for which they braved attack by the armed troops, and witnessed civilians being killed, were later put in the family photo album to which they have now given The Observer exclusive access after 40 years. 'We just realised that these were extraordinary events, and we wanted to set them down,' said Jan.

Taken on 21 August and the fraught days thereafter, they track the invasion's development and depict some of the most dramatic scenes; of the headquarters of Radio Prague being heavily attacked by Soviet forces; of Wenceslas Square and the National Museum, which the Russians mistook for the Communist headquarters, pummelling its façade with bullets (the scars of which can still be seen); and of the funeral of Jan Palach, the Czech student who set himself alight in January 1969 in protest at the Soviet occupation. It is bizarre to see on some of the images how after several days Czechs began to return to their normal business. Women in miniskirts clutch their shopping bags and walk past tanks, apparently oblivious to the troops, as if resigned to the fact that they are in for a long period of frosty oppression. 'We simply didn't understand why they had come,' said Bohumil. 'We were so convinced we had been moving socialism in the right direction with the Prague Spring.'

Neither, recalls Jan, did the troops seem to know why they were there. Many had been told they had been sent to save a country from Western imperialism, but little more, not even where they were. 'I spoke to some of the soldiers in Russian,' said Jan. 'They didn't even know they were in Prague, but they said they had come to fight a counter-revolution. When their commander saw us talking, he pointed a gun at me and told me to scarper.'

During those days the lives of Jan, Bohumil and millions of others, whether they chose to stay or to leave Czechoslovakia, were irreversibly changed. 'We had tasted freedom and the winds of change,' said Bohumil, 'and suddenly it was taken away from us. We were young, so we lost many years of living in freedom.' The frost did not thaw for another 21 years. His most painful memory was hearing the 10 o'clock news bulletin on the radio on 21 August. 'I remember the lady presenter informing us these were her last words, followed by the national anthem. After that the whole family wept together.'

The brothers' friend, Jan Kaplan the film-maker, who never returned to live in Prague after the invasion but made his life in London, has compared the disappointment of the destruction of the Prague Spring to a romantic failure. 'It was like a date for which you were so full of hope. You were all loved up and definitely going to get laid, but then it didn't happen. For Czechoslovakia it was an unsuccessful date with history.'