The biggest protest took place at federation square in Melbourne, where police say more than 100,000 flag- waving people gathered for the main rally.
In Sydney, about 30,000 gathered as at least two big marches converged at martin Place in the city centre while 10,000 more rallied in Adelaide and 20,000 were expected to march in Perth.
Union organisers estimate crowds at rallies in more than 300 cities around the country to reach 500,000.
In an address broadcast to cities and regional centres across the country, Australian Council of Trade Unions (ACTU) president Sharan Burrow told the rally workers must not give up or they would be stripped of their rights as workers.
"Every union leader in Australia is ready to fight like we have never fought before," she said.
"They will strip away what has taken over 100 years to build.”
Stopping the Howard government’s planned legislation was vital to secure the central place of working families in the future, according to ACTU spokesman Greg Combet says the protests are about.
"Working families built this country, they created the nation's wealth, they worked hard to achieve a better life, survived the upheaval of depressions and fought and died in wars for this country - they do not deserve to have their rights taken away by this Government," he said.
Labor leader Kim Beazley addressed about 15,000 people gathered at South Bank Cultural Forecourt in Brisbane, where he issued John Howard a warning “We are coming after you.”
Federal Opposition leader, Kim Beazley, addressed rally in Brisbane telling the gathering the laws are an extreme attack on the rights of Australian families.
"This is about the Americanisation of our industrial relations system," he said.
"These laws - in the intention of this Government - is to push Australian families to the breaking point, to their very limits and we say this to the Prime Minister: 'back off, back off now'."
Roads Blocked
Sydney’s M4 Motorway was blocked by protestors, after more than 3,000 transport workers marched onto the road to sever traffic flow travelling in both directions with 15 freight trucks.
Chanting “John Howard is my name, sweatshop on wheels is my game,” the demonstrators spilled onto the motorway, sitting, waving placards with the words “ Howard’s bill kills” and Jobs with Justice.”
As the rallies crossed Australia, Mr Howard said peole had been misinformed about his workplace changes.
"It is my view that after they (the laws) have been implemented for a period of time, most Australians will look back on the criticisms and objections ... with a sense of bewilderment," Mr Howard said in a speech to the Housing Industry Association in Sydney.
"The sky will not fall in, weekend barbecues will not be abolished, parents will still be able to spend Christmas Day with their children."
But in an ominous sign, several companies have already applied to have protest attendance ruled illegal, enabling them to fine workers or even sue.
Joyce “Listening”
Meanwhile, Nationals Senator Barnaby Joyce- widely considered a crucial swinging vote in the senate, says he will be listening to protesters attending today's rallies.
Senator Joyce is in Canberra sitting on the Senate inquiry into the Federal Government's workplace relations legislation.
He says people have a right to express their views.
"Absolutely, it's their right to protest and I will be sort of listening to the main issues - that's what our job is down here," he said.
Chief executive of the Australian Chamber of Commerce and Industry, Peter Hendy, said more than 95 per cent of workers had “completely ignored” the protests being held around the country.
"They haven't even got anywhere (near) a majority of the union movement membership to turn up, let alone anything more than a fraction of the Australian workforce," he told reporters.
"The ACTU is a joker in the pack. These changes that they are protesting against will actually deliver lower unemployment and higher real wages."