Thousands of people gathered in the Australian capital, Canberra, to protest against a planned carbon tax aimed at curbing greenhouse emissions.
It is the centrepiece of Prime Minister Julia Gillard's environmental strategy, despite her ruling out its introduction during last year's election campaign.
A year on from that pledge, conservative opponents, who opposed the measure, are calling for new elections.
They say the tax will drive up household bills and cost jobs.
No issue in Australian politics is generating more passion right now than climate change, says the BBC's Nick Bryant in Sydney.
This was an orderly protest, but an angry one, with the anger directed at Julia Gillard - "Ju-liar" in the wording of a couple of placards brought to the rally, our correspondent says.
Protesters say this is not only the first day of parliament's spring session but also the anniversary of a pledge that Ms Gillard made during last year's federal election not to introduce a carbon tax.
The proposed tax would penalise 500 of the country's heavy polluters.
It was drawn up after Ms Gillard failed to win an overall majority in parliament at the polls and had to rely on the support of the Australian Greens.
The conservative opposition has promised to ditch the tax, arguing it will have only a small impact on overall global emissions.
Ms Gillard is languishing in the polls and this session of parliament is shaping up to be a season of protest, our correspondent adds.
Next week, convoys of trucks are expected to thunder into Canberra from all over the country in an attempt to close down roads around parliament - again in opposition to the controversial carbon tax.