A crowd estimated at more than 70,000 people on Saturday waved American flags, sang the national anthem and called for the defeat of a Wisconsin plan to curb public sector unions that has galvanized opposition from the American labor movement.

In one of the biggest rallies at the state Capitol since the Vietnam War, union members and their supporters braved frigid temperatures and a light snowfall to show their displeasure.

The mood was upbeat despite the setback their cause suffered earlier this week when the state Assembly approved the Republican-backed restrictions on union collective bargaining rights over fierce Democratic objections.

"I'm deeply honored to be here with you," said Peter Yarrow, a veteran of many social protests during his 50-year folk music career and a founding member of the group Peter, Paul and Mary. "If you persist, you will prevail."

What began two weeks ago as a Republican effort in one small U.S. state to balance the budget has turned into a confrontation with unions that could be the biggest since then President Ronald Reagan fired striking air traffic controllers nearly 30 years ago.

Republicans still must push the measure through the state Senate, which has been unable to muster a quorum for a vote because of a Democratic boycott.

If the plan is approved in Wisconsin, a number of other states where Republicans swept to victory in the 2010 elections could follow. Already, other legislatures including Ohio, Indiana, Iowa, Idaho, Tennessee, and Kansas are working on union curbs.

Unlike previous protests, the rally on Saturday brought out thousands of union workers not directly affected by the bill, including the state's firefighters, exempted along with police from the Republican proposal. Dozens of private sector unions were represented as well at the event.

No "Tea Party" supporters of the proposal championed by Republican Gov. Scott Walker were spotted on Saturday. They staged a smaller rally of their own in Madison a week ago.