The Riverside City Council approved the proposal on Tuesday.
Some gyms were already providing outdoor classes on their own property.
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Others were providing services in parks, but previous regulations called for them to pay fees.
The council also voted to allow churches to host services in parks.
The announcement comes as more businesses across California were allowed to reopen Monday, including barbershops and hair salons, as the state takes a slow and more stringent approach to economic recovery.
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Gov. Gavin Newsom on Friday unveiled a four-tier, color-coded system that will use daily coronavirus case numbers and positivity rates to guide counties' ability to reopen businesses.
All of the counties in Southern California have what is considered a substantial or widespread COVID-19 outbreak.
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