The city said boating, fishing, kayaking, and swimming is not allowed until further notice.
City officials say it's highly recommended that visitors don't swim in the water as exposure to toxic algae can cause rashes, skin or eye irritation, allergic reactions or gastrointestinal problems, and can also be fatal to dogs.
The city is asking to not let pets and other animals drink or go into the water, or near the scum. Residents are being asked not to eat fish or shellfish from the lake, and not to use the water for drinking or cooking.
Boiling or filtering will not make the water safe.
Sampling is being conducted every two weeks by the state for a study, according to the city.
Lake Elsinore has historically been vulnerable to algae blooms, but only began sampling for or learning about Cyanobacteria concerns in 2016, the city says.