The KNSB trade union held a protest outside parliament, demanding higher incomes in the 2027 budget.
After the rally, the unions will drive in a protest convoy along main boulevards and streets in Sofia. It is due to start at about 12:30pm.
Sofia council has announced traffic and parking restrictions because of the march. Prince Alexander I Square is closed to traffic, and police have stepped up their presence there.
Early in the morning, the unions met deputy prime minister and finance minister Galab Donev to discuss next year's budget procedure.
Prime minister Rumen Radev set out the main figures of next year's budget plan on Wednesday. He said the plan will be presented to the European Commission and the Eurogroup.