The protesters have won backing from the former leader of Highland Council.
Dr Michael Foxley claims the increases on the four-minute crossing of the Corran Narrows in Loch Linnhe penalises those struggling to live in the fragile communities to the west.
Dr Foxley, who represented the area for 26 years, said: "There are people living in fuel poverty and extreme fuel poverty. Increasing ferry fares is going to make matters worse.
"The council says there are no equality issues, but the legislation [states] you have to make every effort to ensure you haven't disadvantaged certain people.
"Clearly, fare increases of 25% disadvantages those communities, with the cost of everything from food to fuel going up."
The Nether Lochaber to Ardgour route is used by 290,000 vehicles a year and is a key link connecting the Ardnamurchan and Morvern peninsulas to the trunk road network near Fort William. It is also used by those heading to Mull by way of a ferry from either Lochaline or Kilchoan
Next week's price rise - the third in 12 months - will raise the fares to £7.90 for a single crossing with a car or £69.50 for a book of 30 tickets.
About 90 people attended a public meeting in Strontian on Sunday, and a working group has been established to consider options for the type of direct action.
A Highland Council spokesman said the authority had to bridge the gap between the income generated from fares and the cost of running the ferry, which provides a 15-hour per day shuttle services.
He added that income fell £173,000 short of the annual running costs, estimated at £1.3 million.