May says she supports efforts by Community Coalition Against Mining Uranium, which opposes a potential uranium mine in eastern Ontario.
She said a mining firm, Frontenac Ventures, is prospecting for uranium on Algonquin First Nations territory near Sharbot Lake, west of Ottawa.
"Canada must stop mining and refining uranium," said May.
"The uranium extraction process is extremely hazardous to the environment and to the health of mine workers and the public."
The hazard goes well beyond the mines and refining plants, she said.
"Radioactive particles carried downwind and downstream have the potential to poison thousands of eastern Ontarians through the air they breathe and the water they drink."
The industry also fuels the nuclear arms race, she said.
"Uranium mining and nuclear power are the greatest obstacles to the goal of global nuclear disarmament. Mined uranium inevitably ends up as plutonium, radioactive waste, or worse nuclear weapons."
May chastised the Ontario Liberal government of Premier Dalton McGuinty now embroiled in an election campaign for its plan to build more nuclear power generating stations.
She called that a foolish decision, taken by a government acting like "bunnies in the headlights."
"The McGuinty government has completely and utterly failed to grasp the potential of energy efficiency and conservation."
Ontario's plan to build more nukes has raised the demand for uranium, she said. "This pressure for uranium mining in Ontario... is part of this whole new nonsense that we're going to have more nuclear power."