The Oregonian reported that the development halts the city's plans to add fluoride to Bull Run water, which serves the city and other customers -- about a quarter of the state.
Clean Water Portland's drive needed only 19,000 valid signatures but got more than 33,000.
The group said it plans an initiative campaign to put more restrictions on the city's fluoride efforts.
A leader of the group promoting dental health and backing the City Council's vote expressed confidence Portland voters would get accurate information and support fluoridation.
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