Bristol's mayor wants to raise the cost of business parking permits by up to 400%, the Liberal Democrats say.
The party said it would oppose George Ferguson's plans and claim it would have a "devastating" effect on trade.
Under the proposal business owners could have to pay up to £500 for one annual customer permit with residents paying between £48 and £192.
Mr Ferguson said: "The whole point is to reduce the amount of traffic on the roads by commuters at peak hours."
He said the volume of traffic "was making the roads more difficult for the rest of us" and said he was open to all responses except on whether the permit scheme should go ahead.
At present the scheme operates in the Cotham, Kingsdown, and central areas.
Mayor Ferguson wants to roll out 18 residents' parking zones (RPZ) within the next 18 months.
The Liberal Democrats said they will throw their weight behind calls for the project to be redesigned.
Councillor Christian Martin, the group's transport spokesman, said the figures were "daylight robbery" and "would be opposed all the way".
"Mayor Ferguson must tell his advisers to go back to the drawing board and start again," Mr Martin said.
"The principle of RPZ is fine, in areas where people want them. But this one-size-fits-all scheme - which is being imposed on people whether they like it or not - is nothing less than a parking tax."
No cost apology
Mr Ferguson wants to roll out the scheme, which is currently active in three areas, to much of the city and believes any area opting out would suffer with an influx of people looking for somewhere to park.
He added he made no apologies for the cost of the permits and said the money was needed to set up the scheme and to improve public transport.
"They're probably about a quarter of what the commercial charge for a parking space would be - albeit not a dedicated space," Mr Ferguson said.
"We're talking about an absolute fraction of the overall costs of running a car - it's also a fraction of the cost of running a business."