The protesters, who came from across New Brunswick, were carrying signs with slogans, such as "Broken contract equals broken trust," and "Ditch the dictators."
The retired public servants say they will not agree to the proposed reforms, which are tied to the government's plan to move to a shared-risk model for the public service.
Finance Minister Blaine Higgs met with some of the angry protesters and told them the existing plan has to change.
"What we have — if you wanna listen for one second — what we have found here is a pension plan that this province cannot afford. It is not sustainable," Higgs said over shouting.
"We're going to lose it for you, for the current employees, and future employees. That's why we're doing it," he said.
Higgs told reporters the protest will not prompt further concessions on the provincial government's pension plans.
"Absolutely not," he said.
The Alward government signalled during Tuesday's throne speech it plans to press ahead with pension changes and will introduce legislation this session.
"The status quo is not an option," the speech stated.
The pension plan is currently facing a $1-billion deficit, which is expected to grow. The government wants to move to a shared-risk model so taxpayers don't have to top up the fund in years when it loses money.
Higgs did say earlier this week, however, that the government was softening its stance on pension reforms for retirees.
He said a letter was being sent to retirees, promising that their base benefits will never be lower than they currently are.
But the retirees say they do not trust the government.
They are also upset about the lack of an absolute guarantee of annual cost of living increases.
If the markets perform poorly in a given year, there may be no cost of living increase. But the government says it will be made up in years when the markets perform well.
Over a 20-year period, the government says the average cost of living increase will be at least 75 per cent of the rate of inflation, but says it has run the economic statistics from the past 20 years through the reformed model and there would have been a full cost of living increase in every one of those years, even during the 2008 market crash.
Still, civil servants who have already retired have said they would fight any change in benefits in court, arguing it's not fair to change their benefits retroactively.
Under the current plan, retired civil servants are sheltered from any risk of market downturns by the provincial government with guaranteed cost-of-living increases.
Under the reforms, announced by Premier David Alward in May 2012, the risk would be shared by both sides.
The proposed model also includes increased contribution levels and higher age of retirement phased in slowly.
The size of the anti-pension reforms protest exceeded Tuesday's anti-shale gas demonstration.Suggest a correction