Amanda Bynes has been placed on a psychiatric hold after she was found roaming the streets naked and alone ... TMZ has learned.

An eyewitness tells us ... Bynes was seen walking near downtown Los Angeles early Sunday morning without any clothes. We're told Bynes waved a car down, telling the driver she was coming down from a psychotic episode. Amanda herself then called 911.

Our law enforcement sources say Bynes was taken to a nearby police station, where a mental health team determined she needed to be placed on a 5150 psych hold.

Fortunately, our sources say it doesn't appear Bynes was hurt at all during the ordeal, though things could've been much worse given the area where she was found.

A source close to Amanda tells us she's currently hospitalized, and will likely continue to receive care for several days. Typically, a psychiatric hold only lasts 72 hours, but it can be extended.

It was almost one year to the day when Amanda's 9-year conservatorship was officially terminated. Amanda's mother was her conservator starting back in 2013 when it was clear she was having issues with her mental health -- she was diagnosed with bipolar disorder. After a series of bizarre incidents -- including one where she set her neighbor's driveway on fire, almost engulfing her dog in flames -- she was sent to a psychiatric ward for treatment.

Interestingly enough, Amanda was supposed to attend 90s Con in Connecticut this weekend, an event she canceled on due to illness.

Since the conservatorship ended, Amanda seemed to be doing well for herself, though the most recent incident is obviously very concerning ... it's unclear if her parents or legal team will step in.