In the biggest demonstration in the country's post-war history, protesters held up banners and chanted: "We want democracy now."

Other demands included free elections, a new constitution and the dismissal of the remaining hard-line members of the Politburo.

The gathering, in the city's Aleksandr Nevsky Square, comes just eight days after the country's Communist leader, Todor Zhivkov, 78, was ousted from power following a 35-year regime.

He was replaced by the more moderate former foreign minister Petar Mladenov, 53, who has promised reform.

'Democracy and pluralism'

Most of Zhivkov's loyal supporters have already been dismissed and the newly-formed Parliament moved quickly to repeal a repressive law against freedom of speech which had previously led to the imprisonment of thousands.

Today's protest, organised by dissident political groups, included many of the country's academics and literary personalities who had been banished under the Zhivkov regime.

Radoi Ralin, a once-imprisoned poet, said: "We want democracy and pluralism.

"We want freedom of people's opinion, freedom of people's speech, freedom of people's will.

But he also signalled a note of caution warning that the new leader may not be as good as his word: "For years we have been promised radical changes in our society, but it always turned out to be a carnival in which masks were changed but policy remained the same.

"That is why we should not be too enthusiastic about the latest changes. We have to see what the new leaders have to offer us soon."

Numerous similar demonstrations have taken place across Eastern Europe since the collapse of Communism in the Soviet Union.

Bulgaria has been one of the countries most resistant to change. Just two weeks ago Mr Zhivkov issued a statement stressing that the Bulgarian Communist Party was still in total control.

But as the ideals of "perestroika" and glasnost" swept through countries including Poland, Eastern Germany and Hungary, Mr Zhivkov's grip on power became increasingly weakened.

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