Rick Clarke, president of the Nova Scotia Federation of Labour, said the unions were blindsided by the Liberal government after it unilaterally decided to make the changes following a summer of negotiation.
"My mother had brought me up never to call anybody a liar, but I have never been misled by a group of people as severely as we have been by these folks," said Clarke on the steps of the legislature.
He was cheered by about 100 supporters who at one point during proceedings broke into a chant of, "My vote, my union, my choice."
Joan Jessome, whose Nova Scotia Government and General Employees Union represents most of the health care workers that would be affected by the legislation, said there was no way the unions would bargain away the rights of their membership.
"NSGEU has 14,000 health care workers impacted by these decisions," said Jessome. "There is no way in good conscience could I sit down and negotiate away benefits for members and they have no say."
The unions said they want to form a bargaining association to avoid splintering their members in contract negotiations.
But Premier Stephen McNeil said that proposal still leaves far too many bargaining units to deal with and the collective bargaining process needs to be streamlined so that the government isn't perpetually negotiating contracts.
"What they brought to us in my view was the status quo," McNeil said.
He also said the unions should wait to see the legislation when it is introduced Monday before arriving at conclusions.
"I'm not sure how they feel they've been misled," he said. "The piece of legislation will reflect what was talked about with them."
Health Minister Leo Glavine said Thursday that the government decided to impose a framework that will identify which unions will represent nurses, technologists, administrative and support workers after negotiations with the unions broke down over the summer.
On Friday he appeared to back away from that statement, albeit slightly.
"The legislation doesn't specifically say what union people will be in," said Glavine. "It outlines a process that will enable to get to a streamline position."
Some of the workers who attended the rally were clear that they want to stay with the unions that represent them now.
Lynn Stanton, a Halifax nurse who works at the Victoria General Hospital's intensive care unit, called the government's move "shoddy."
"In our union we fought long and hard for the benefits that we have gotten," she said. "I've put in over 28 years there and I want to stay where I've been."
The government has repeatedly butted heads with labour since sweeping to power nearly a year ago.
Earlier this month it was named in a court challenge that aims to overturn essential services legislation that ended a strike by 2,400 nurses in Halifax in April.
Following the rally Friday, Clarke said the unions will discuss their options over the coming days, including the possibility of further legal action.