Angry marchers surrounded the state television station, whose output is is tightly controlled, and demanded air time for opposition politicians. The demonstrators left at the urging of police, but riot police later beat groups of protesters outside the headquarters of the security service, which is still known as the KGB.
The demonstration was originally called to mark the founding of the Belarussian People's Republic in 1918, which only survived nine months before the country was carved up between Poland and Soviet Russia.
But the purpose of the rally changed after Saturday's announcement in Moscow of a new "union" between Belarus and Russia. The declaration was made by the Belarussian President, Alexander Lukashenko, with the Russian Prime Minister, Viktor Chernomyrdin, standing at his side.
The union, which the Kremlin insists does not mean any change in sovereignty, came as no surprise. Mr Lukashenko has made no secret of his desire for integration with Russia. He has said the terms will include setting up a "Supreme Council", comprising the two presidents, the prime ministers and parliamentary leaders.
The pact, to be signed on 2 April, is a measure the post-Soviet economic decline of Belarus and its weak sense of national identity compared to other ex-Soviet republics, many of which seek closer co-operation with Russia but balk at moves that smack of a return to Moscow's rule. It is likely to worry Ukraine, which will not welcome signs that Russia is poised to devour its western neighbour.
Russia has long been aware that Belarus lies on a key route between Moscow and Berlin. But the pact appears to have as much to do with an attempt to win votes by President Boris Yeltsin. With 12 weeks to go before he faces the electorate, he is mindful of the nostalgia for Soviet times.
This was driven home on 15 March, when the State Duma (lower house of parliament) caused a stand-off with the Kremlin by denouncing the accords that dissolved the Soviet Union and created the Commonwealth of Independent States (CIS).
Many CIS states viewed the vote as alarming, fearing their independence may be threatened if the Communist Party, which has the largest number of seats in the Duma, has a president in the Kremlin.
Mr Lukashenko does not share these concerns, but many of his liberal parliamentarians do. They were furious they were not consulted, before he went to Moscow to give away a portion of their freedom.
Mr Lukashenko, a former collective-farm boss, displays some of the worst instincts of Soviet Bolshevism, from censoring the press, to banning trade unions and ignoring his own constitutional court. He has been pressing for further integration with Russia since his reelection in 1994, and backed attempts to inhibit the revival the Belarussian language. Mr Lukashenko also supported the referendum which replaced the country's red-and-white national flag with a Soviet-style standard.
"Lukashenko has ceased being a president. He is now outside the law and the constitution," Zenon Poznyak, leader of the Belarussian Popular Front, told the crowd on the streets of Minsk.