On 20 August 1968, the Prague Spring - in which Prime Minister Alexander Dubcek tried to bring in liberal reforms - was crushed by Soviet tanks. It would take more than 20 years before the country would be free from communist rule.
Here, three BBC website readers share their memories of the invasion and life under Soviet occupation.
DR PAVEL GROSSMAN, ABINGDON, UK
I had just finished playing bridge late one night with fellow lecturers at our hostel and had gone to bed at about 1am. An hour or so later, a colleague rushed into our room and shouted that the Soviets had invaded.
We thought he was being ridiculous and he convinced us to listen to the radio to hear for ourselves. Sure enough, national radio was broadcasting the news.
Working at the University of Kosice, in eastern Slovakia, we quickly realised that we were likely to be among the first places that the Soviet army would reach. By about 4am, we saw the first tanks rolling into town.
The Prague Spring had given everybody a real hope of free and creative life in a more democratic society. The Soviet invasion crushed this hope.
For several days and nights I continued to watch as the bulk of the Soviet occupation forces thundered through the town - a terrifying experience.
A poster war was launched during the first days of occupation. The Russian posters were usually taken down by the early morning shift going to work.
One morning, walking through the park to my department, I noticed a Russian poster still up on a tree. Angry, that other passers by left it there, I ripped it to pieces and put it in the bin.
Only then did I notice a group of Russian soldiers in an armoured carrier nearby staring in disbelief at me. They quickly knocked me down to the ground and their officer pulled his pistol from its wooden holster.
I thought my last moment had come. Luckily, after some tense negotiation when I supplied a fictitious name and address, they let me go.
The invasion made it finally clear to those who still needed convincing that communism and democracy are incompatible.
It also made it clear that the situation in Czechoslovakia wouldn't change until it changed in Russia itself and that, on past evidence, it could take another 100 years.
This terrible realisation drove hundreds of thousands of people, like myself, to seek new life abroad. I was one of the last people to manage to leave Czechoslovakia legally, as I already had a pending visa to go to Norway.
The day after I arrived in Oslo, all exit visas were cancelled.
Fortunately, the change in the Soviet Union didn't take a century, only a generation, and since 1989 I have been able to enjoy visiting my family and friends in a free and democratic Czechoslovakia and later the Czech Republic.
DANA BRAZDOVA, DECIN, CZECH REPUBLIC
I was 17 years old in 1968 and I had one year to go before my final exams at a secondary grammar school.
I was interested in what was happening in society and was very excited by the changes in the political climate.
I had high hopes for the future. It was like a breath of fresh air after the previous oppressive years. My parents had experienced Stalinism in the USSR and therefore did not share my enthusiasm - they were sure the Soviets would come.
In the night of 20th August, tanks were driving through the border town where we lived at the time. Airplanes were flying over one after another and our house was shaking.
We thought these were just training manoeuvres and only later in the morning did we learn the truth.
After overcoming the initial shock and fear of what had happened, I still had a naive hope for several months that the people, who had after all just experienced what freedom was, would not bend to the Soviets. The reality, however, was different.
The totalitarian regime's best weapons were isolation and lack of information. It was not such a problem for people in large cities as it was for those living in smaller towns and villages.
I felt isolated, as I could not speak freely with other people except for a few close friends.
The economy continued to deteriorate, with food shortages being commonplace.
In the days leading up to the fall of the communist rule in 1989, I took part in the mass demonstrations. I did not leave the protest line as I felt I had to stay for the sake of our children.
In the next few days the euphoria came, with a sense of pride that we had not bent to Soviet strength. However, it would take years before I overcame the worry that those former times would return.
MARCELA SOUKUPOVA, ST ALBANS, UK
My parents never talked about the political situation. We would sometimes listen to music from composers that I later discovered was banned by the government.
One day I started singing some lines of this song while we were out in the street and my father slapped me. He never told me why - although I understand the reason now.
I would also find out years later that my father had tried very hard to persuade my mother to take me and my sister out the country.
But my mother's love and concern for her parents prevailed and she decided to stay to look after them. I had no idea how desperate my parents were at the time.
I also later found out that my Mum and Dad were scared to talk freely in front of me and my sister, because we were being indoctrinated in school and our love for Lenin was probably bigger than that for our parents.
I still remember those days and how proud me and my sister were of communism and the state because of the songs and stories we were taught at school. We would even sing those songs at home because we loved them so much.
When the people went to the streets in 1989 to "ring the final bell" for communism with their keys, my parents stayed at home for the fear that it would never be over and if they were seen in the streets it would have repercussions on them in the future.
My parents lived under communist rule for 21 years and the idea that somehow it could be over was completely unrealistic for them. I was only nine years old at the time.
Now I have been living for over 20 years in a free country and it seems almost impossible that such things had happened. I only hope that history will not repeat itself.