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.

United Voice, the union representing some of the country's lowest-paid workers in industries such as hospitality and aged care, told the crowd that "we deserve a government that supports us, not hurts us".

"We are out on the streets today because were angry at a government who wants to make us work harder and work longer for less money," state secretary Jess Walsh said.

"Any were angry at at a government who wants to make us only be able to go to the doctor if we can afford to pay . . . who wants to deny our children to go to university, to better themselves and to have hope for the future.

"And looking around the streets of Melbourne today, I don't think we are alone."

Kevin Bracken, of the Maritime Union of Australia, led a minute's silence in honour of dock worker Anthony Attard, 42, who was crushed to death at the Port of Melbourne last month.

"The government's attack on red tape is an attack on safety," he said.

Trades Hall estimates the crowd has topped 20,000 protesters.

Representatives of the CFMEU, the National Union of Workers, the Electrical Trades Union, the Plumbers Union, the Maritime Union and the Australian Services Union were among those leading the march.