Shouting anti-government slogans and carrying banners calling for the laws to be scrapped, more than 100,000 workers took to the streets on Wednesday and blocked traffic in the main cities of Sydney and Melbourne.
The government passed the laws late last year, saying they would improve productivity and weaken union influence in the workplace by encouraging workers to sign individual work contracts and abandon union-based award conditions.
The union movement and opposition Labour Party say the laws cut wages, make it easier for employers to sack workers and decrease job security.
The laws have re-energised Australia's union movement, lifting the union-based Labour party in the polls after a decade in power by the conservative government of the prime minister, John Howard.
Labour leader Kim Beazley, speaking to the rally of more than 80,000 workers in the southern city of Melbourne, said: "This is a battle for ordinary Australian life. This is a battle for Australian families."
The government said the turnout would have disappointed the unions, as millions of Australians went to work as normal.
"The majority of the 10 million workers in Australia will go to work today and will get on with their jobs because they know there are more jobs and higher wages as a result of the reforms over the last decade," said Kevin Andrews, the workplace minister.
Beazley has promised to scrap the laws if his party wins power at the next elections, due in the second half of 2007.
Labour has been ahead of the government in the polls for four of the past six months, although Howard reclaimed a narrow 51- to 49-point lead in mid-June after announcing tax cuts.
A Newspoll in mid-June found Beazley's Labour more popular on social issues, and ahead 48% to 29% on the question of who could best handle workplace laws.
A poll in the Age newspaper on Wednesday said some of Australia's biggest companies had shunned the laws, preferring to promote company-wide employment agreements.