More than two dozen groups organized the Rally to Save the American Dream in big cities and capitols in all 50 states.
Several hundred demonstrators gathered in front of New York's City Hall to show their support.
They carried signs with slogans like "Unions Make Us Strong" and "What Happens in Wisconsin Doesn't Stay in Wisconsin."
Dozens wore Cheesehead hats or jackets showing their membership in unions like the Teamsters.
The demonstrators say they fear the end of collective bargaining for all workers if Wisconsin's governor succeeds in an effort to weaken unions for public servants.
New York Rep. Anthony Weiner told the crowd that workers were being unfairly scapegoated for the "crimes that happened down in the financial district."
A separate rally was planned for the state capital.
The rallies are intended to show support for union workers in Wisconsin. Gov. Scott Walker wants to end nearly all collective bargaining with public employee unions to help balance the state budget. The Republican Assembly passed his proposal after a battle lasting more than a week. Senate Democrats who oppose the plan but lack the votes to stop it have fled to Illinois to keep the vote from taking place.
New Jersey unions and supporters demonstrated in Trenton on Friday.
"This is a defining moment for the labor movement and our middle class," said state AFL-CIO President Charles Wowkanech.
"We are one, we are united, and together we will fight for economic and social justice and our shared democratic values. We must send a strong message that union busting is not a budget solution and that we need to improve the quality of life for all workers - not dismantle it."
Stephanie Bloomingdale, secretary-treasurer for the Wisconsin AFL-CIO, said her state is ground zero in a nationwide attack on union workers. She and two other Wisconsin union officials flew to New Jersey for the unity rally.
"Our fight is New Jersey's fight," said Bloomingdale. "And New Jersey's fight is Michigan's fight. And, Michigan's fight is Ohio's fight for America to reclaim our middle class."
New Jersey Gov. Chris Christie, who was out of state Friday, has proposed raising the retirement age and requiring public workers to pay more for pension and health benefits. He said he supports Walker, a fellow Republican, for taking strong actions to balance the state budget but says he supports "responsible" collective bargaining.
"What's up with your governor, anyway?" AFL-CIO president Richard Trumka asked to loud applause. "What makes him claim that middle-class working people are the haves and you are somehow robbing from the taxpayers?"
Barbara Keshishian, head of the state teachers union, which has often been the subject of Christie tirades over the past 14 months, described the current situation between the governor and the unions as a "war."
"Make no mistake about it," she said, "We are in the middle of a well-funded, well-orchestrated war on organized labor and public education. It is beginning with public-sector unions, but the ultimate goal is to destroy all labor unions."
Union leaders view Christie's proposals as attempts to weaken organized labor, which can be a powerful ally to Democrats. All 120 legislative seats are up for re-election in New Jersey in November, and Christie has made no secret of his desire to wrest control of at least one house from the majority party.
Asked about the rally during Thursday night's "Ask the Governor" call-in program on Millennium Radio's 101.5 FM, Christie said the protesters "should stay in Wisconsin."
"What we're doing is trying to put New Jersey back on the path of fiscal soundness," he said. "Their problem is, they don't want to pay their fair share."
Earlier, Sen. Michael Doherty, a conservative Republican who spoke at the tea party rally, said New Jersey must pass pension and health care changes or the underfunded systems will collapse. He said he believes public employees are paid more generous salaries and benefits than their private-sector counterparts.