The Construction, Forestry, Maritime, Mining and Energy Union led the rally from Belmore Park to Hyde Park on Thursday.

An estimated 5000 people marched, blocking several city streets at lunchtime.

"This is what breaking unjust laws looks like," maritime union official Paul McAleer told the cheering crowd.

"This government has to realise that the extreme laws they've put in place are not enough and not big enough to tame us. There is not a law big or ugly enough in this country for us not to fight it or for us not to break it."

Electrical Trades Union national secretary Allen Hicks said the march sent a message about the daily "discrimination" building and construction workers face.

The Australian Building and Construction Commission and new building code restricted unions' ability to negotiate wages and conditions, he said.

"Enough is enough."

Master Builders Australia said the protest was "regrettable" because the ABCC was designed to protect workers who aren't union members along with small business owners, sub-contractors and tradies.

"While the CFMMEU chooses to flout the law the victims of its bullying and the wider community want the union to devote the same effort to obeying the law like everyone else," chief executive Denita Wawn said in a statement.

"Sadly the CFMMEU chooses the opposite."

Prime Minister Scott Morrison this week threatened to deregister the CFMMEU after voicing his disgust with a Victorian official using his children to campaign against the building industry watchdog.

Australian Associated Press