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Several members of the liberal Czechoslovak leadership have been arrested, including Prime Minister Alexander Dubcek.
The Soviet news agency, Tass, claims "assistance" was requested by members of the Czechoslovak Government and Communist party leaders to fight "counter-revolutionary forces".
But in a secret radio address, Czechoslovak President Ludvik Svoboda condemned the occupation by Warsaw Pact allies as illegal and committed without the government's consent.
US President Lyndon Johnson said the invasion was a clear violation of the United Nations Charter and that the excuses offered by the Soviet Union were "patently contrived".
"It is a sad commentary on the communist mind that a sign of liberty in Czechoslovakia is deemed a fundamental threat to the security of the Soviet system," he said.
The Czechoslovak authorities have ordered their vastly outnumbered army not to fight and are appealing to the public for restraint.
Czechoslovakia's abortive path to freedom began when Mr Dubcek, a Slovak, became Communist Party leader in January.
A programme of wide-ranging democratic reforms had been gathering pace in the face of Soviet disapproval and the rebirth of social and political freedom became known as the "Prague Spring".
In the capital of Prague today, crowds of people gathered in the streets chanting support for Mr Dubcek and imploring the foreign troops to go home.
Much of the resistance was centred around the Prague radio station. As the day progressed, Czechoslovak youths threw home-made missiles and even tried to take on Russian tanks.
Reports say some tanks and ammunition trucks were destroyed, but Soviet troops responded with machinegun and artillery fire and at least four people were shot dead.
In the Wenceslas and Old Town Squares, hundreds of youths made barricades out of overturned lorries to try and halt the advance.
Soviet and eastern block commanders have now imposed an overnight curfew and are threatening to shoot on sight anyone caught breaking it.
All rail, road and airline routes out of Czechoslovakia have been closed as troops continue to enter the country - now estimated to number nearly 175,000 men.
Following the invasion, Mr Dubcek and others were banned from office and replaced with a famously repressive communist regime. All the reforms were annulled or abandoned.
The invasion drew condemnation from across the globe. Significantly many western communist parties and communist Yugoslavia and Rumania dissociated themselves from the USSR's actions.
As with Hungary in 1956, the West took no action. The Soviet defence minister is said to have recommended invading even if it meant third world war. Moreover, the US was in the middle of a presidential election campaign and entrenched in Vietnam.
The Communists were finally ousted on 24 November 1989 and Mr Dubcek made a triumphant return to Prague.
He became chairman of the new post-Communist administration in what became known as the "Velvet Revolution".
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