The county council has agreed to cuts of £665,000 to the fire service budget.
The chairman of Lincolnshire Fire Brigade Union, Ben Selby, who lives in Grantham, said the cuts would put lives at risk.
Lincolnshire County Council agreed a package of measures today while protesters waited outside the council offices.
Mr Selby told the Journal: “These cuts will put the lives of our members and those of the public at risk right across Lincolnshire. Our campaign will not stop here. We will have to take it up through legal action with health and safety legislation in mind.”
The union says the cuts will mean fewer firefighters will man each fire engine making it more difficult to fight fires and reducing response times.
Chief fire officer Dave Ramscar said: “Obviously, in a perfect world we wouldn’t have cuts to the fire service.
“But I am confident that the consultation has been carried out correctly and there’s going to be the least impact on risk for the residents of the county of Lincolnshire.”