Kettering Extinction Rebellion say offensive graffiti between their art is nothing to do with them.
The activists have put up art work around Kettering to raise awareness of the climate crisis.
However, some written graffiti has appeared between the posters left in Meadow Road, just off the High Street, and Kettering Extinction Rebellion say this has nothing to do with them.
One of the messages reads: "Tories not welcome". Another says: "Kill the rich".
The group said: "We had nothing to do with the spray paint graffiti on the Meadow Road hoardings. (The messages) are not fitting with XR principles and values and were there before the art work went up."
Kettering's MP, Conservative Philip Hollobone, said: "All graffiti is awful, it's a criminal offence.
"All graffiti just brings our town down, it's only a few people who do it and I hope it's cleaned up as soon as possible."
Mr Hollobone added: "All graffiti is awful, it doesn't matter what it says."
Extinction Rebellion's art has a positive focus and a spokesman said: "Extinction Rebellion Kettering's art boards inject colour, creativity, and vivacity into out town and encourage people to see environmental activism as a power for good."
They said the art was a force for good culturally "by jazzing up Kettering's tired facades, vacant buildings, grimy vanity screens and boarded up windows".
The group said: "The art boards, installed by guerrilla-style by XR Kettering, are a necessary reaction to the lack of urgent action taken by Kettering Council to tackle climate change and reduce biodiversity loss in the wake of its climate emergency declaration last July.
"XR Kettering hopes residents will enjoy the art created by its members, reflect on their meaning and question why our council is doing so little to safeguard our collective futures."
The group have left 24 board on hoardings and vacant buildings in Meadow Road, Rockingham Road and London Road.