A sea of high-vis workwear swamped Melbourne's CBD on Thursday as several thousand protesters joined to march against the Abbott government's budget.

Unions representing workers from tradesmen and stevedores to firefighters and nurses gathered at Trades Hall on Lygon Street shortly after 10am, before rousing speeches blared through megaphones and the crowd headed to Parliament House via Swanston Street, blocking traffic at several city intersections on its way.

Holding hundreds of flags and placards emblazoned with union slogans and messages against the hard-line budget and the federal government's "war on red tape", the crowd descended on Spring Street as part of the "Bust the budget" rally.

"They say cutback, we say fight back," the protesters chanted.