Thousands of construction workers rally in Melbourne and Sydney ...

Thousands of trade workers have walked off the job in Melbourne and Sydney to protest against the federal government's decision to force the CFMEU's construction arm into administration.

A sea of orange and yellow high-vis vests and hoodies formed outside Trades Hall on the edge of Melbourne's CBD, with workers waving CFMEU flags and chanting.

The Sydney rally began at Belmore Park, with members marching towards the state parliament house on Macquarie Street.

This is the second time the CFMEU has rallied in Sydney in recent weeks. (ABC News: Keana Naughton)

The federal government intervened last month in response to allegations of corruption and criminal infiltration into the union.

Media coverage of the allegations had led to Victorian branch secretary John Setka leaving his role in July. 

The federal government is planning a meeting next month between unions, government and businesses, in a bid to reset in the wake of the CFMEU scandal.

CFMEU secretary Zach Smith urged the crowd to unite against the government and administrator's plans for the union. (AAP: James Ross)

In Melbourne, CFMEU national secretary Zach Smith raised concerns that the appointment of the administrator would lead to a reduction in wages and conditions.

"An attack on one union is an attack on all," he said.

"Our victory is assured if we stand together and fight together and today we will be sending a very clear message across those corporate board rooms, across the halls of parliament that we're drawing a line in the sand."

Major Melbourne thoroughfares were blocked to traffic and several tram and bus routes were disrupted as the workers marched through the CBD.   (ABC News)

The Electrical Trades Union's Victorian secretary Troy Gray told the crowd the appointment of the administrator could put 100 years of union achievements at risk.

"60,000 workers today are saying we're not taking a backward step, not today, not now, not ever."

Mr Gray conceded there were likely to be bikie members in the building industry but said they probably numbered no more than a dozen of the state's 300,000 construction workforce.

ETU Victorian Secretary Troy Green led today's rally, telling the crowd to fight against any changes to the CFMEU. (ABC News)

"One of the proud things about the construction industry is that we give people a second chance," he said.

"The average wage for a Canberra politician is $240,000 and yet they put your union into administration and you only earn 150-grand and you work a lot harder than the lot in Canberra."

The workers cheered as Mr Gray defended Mr Setka, who he said he had worked alongside for around 30 years.

"In those years that I've worked in the CBD, there was no better union official than John Setka," Mr Gray said.

Speaking at the Sydney rally, delegate Denis McNamara said he believed the government's intervention was unlawful.

"The federal government have taken over our union. They've put our union in administration with no proof," he said.

"You know, there's been allegations from the media, and from that, they've decided to put our whole union into administration."

Thousands of people took to the streets in Sydney, starting in Belmont Park and marching towards state parliament. (ABC News: Keana Naughton)

Ousted CFMEU New South Wales secretary Darren Greenfield addressed the Sydney rally.

Mr Greenfield, who had to step down earlier this month, accused the government of a campaign against the union.

"I will always … speak up for what is wrong, always for our members, we will always stand up and defend it," Mr Greenfield said.

"This campaign … by this rotten Labor Party and the ACTU on our union is coming for every union in this country."

Protesters in Sydney were told the administration process had been unfair. (ABC News: Keana Naughton)

Today's rallies follows marches that included tens of thousands of members of other trades unions, including the Australian Manufacturing Workers' Union, Electrical Trades Union and the plumbers' union, in Melbourne, Sydney, Adelaide, Brisbane, Cairns, Canberra, and Perth just weeks ago.

Police have urged those attending rallies to protest peacefully.

The Fair Work Ombudsman has advised employers to consult with workers on whether they intended to use available leave if they were absent from work today.

Employers have been instructed to dock a minimum of four hours' wages from those engaged in today's action, as set out in the Fair Work Act.