The Welshpool route, known as the 'Wiggly Path', which runs alongside the boundary to the now-closed Ardwyn School, links to the Red Bank area of the town.
Local councillor, Graham Breeze, is one of those calling on Powys County Council to re-think the decision.
He said: "It is really disappointing that Powys County Council has erected signs telling the public that they no longer have access to the famously named Wiggly Path.
"The footpath has acted as access for pupils and parents for over 60 years as well as a valuable link to the Red Bank area of the town. I have personally used the path for over 30 years.
"The Red Bank area is a particularly dangerous road for pedestrians with limited pavement areas."
Councillor Breeze said that in 2017 the council attempted to close the path citing safety concerns at the school but, he said, they were persuaded to improve boundary fences instead after residents demonstrated on the site and he challenged the decision.
"Now signs have gone up without prior warning."
"Concerned residents are bombarding me with complaints," he said.
"The closed school has its own boundary fences in place so there should not be an issue with this pathway running around the outside."
A spokesperson for Powys County Council said: “The council is taking steps to secure the former Ardwyn Nursey School site whilst the site is being decommissioned and to ensure that public safety is being maintained during this process.
“The council recognises that many residents use this path from the town to Red Bank and no physical barriers have been placed to prevent access to the path. However, the path is not recorded as a public right of way. The future of the site will be decided in accordance with the council’s Corporate Asset Policy.”