Their protest came after the ruling Bulgarian Socialist Party (BSP) refused to vote on a "salvation declaration" proposed by the opposition Union of Democratic Forces (UDF) that called for an early national poll.
But parliament began an extrordinary session to discuss the opposition's declaration and voting started on the proposal. The opposition says the BSP has failed to tackle the country's economic crisis and lacks support to form a government.
While demonstrators in Serbia have now taken to the streets for more than 50 days in succession, their counterparts in Bulgaria only began their pro- tests last week, drawing ever increasing numbers each day.
UDF leaders, in part inspired by events in Belgrade, hope their protests will gather a similar momentum and are trying to draw on similarities in the de-monstrators' cause. "We protest against one and the same discredited Communist regimes," said UDF leader, Ivan Kostov.
Although both Bulgaria and Serbia are ruled by ex-Communist Socialist parties, there are important differences in their protests. Whereas the Serbian demonstrators seek a reversal of the government's cancellation of opposition local election victories, the Bulgarians are trying to oust a government which, while un- pop- ular, was legitimately elected to power in December 1994 - and which still has two years of its mandate to run.