Hundreds of dockers and oil refinery workers in the Black Sea ports of Varna and Burgas downed tools in what was expected to be the first of a wave of strikes. Thousands of miners are expected to follow suit today with tens of thousands of workers nationwide joining in tomorrow.
With the demonstrations in Bulgaria only entering their second week, the enlisting of trade-union support marked a significant triumph for the opposition.
The Bulgarian economy - with annual inflation running at more than 300 per cent and average monthly salaries down to the equivalent of pounds 12 - is in a perilous state and a full-blooded general strike would be disastrous.
The Socialist Party last night appeared keen to compromise. After a crisis meeting of the party leadership, a statement said that the party would be prepared to discuss an early general election with the opposition and that it had drafted a programme aimed at bringing the country out of its political and economic crises.
The offer echoed Sunday's proposition made by Socialist Party leader Georgi Parvanov that elections, which are not due for almost two years, could be brought forward to this year in return for opposition support for another Socialist-led government in the medium term.
In addition to calling for strike action, opposition leaders are also planning to stage daily street demonstrations, modelled along the lines of those in Serbia.
Tens of thousands of people were again on the streets of Sofia yesterday, though unlike the protests on Friday, when more than 100 were injured in clashes with riot police, the demonstration passed off peacefully.