It happened across from Lake Merritt and Children's Fairyland around 11 a.m., police said.
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The man is described as 25 to 30 years old and is about 5 feet and 10 inches tall and weighs 140 pounds.
Police said the suspect forced the woman into the building's restroom where the sexual assault took place.
Anyone who recognizes this man is asked to contact Oakland Police Department's Special Victims unit at 510-238-3641.
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