Amanda Bynes -- Hospitalized On 5150 Hold

8:35 AM PT -- TMZ just spoke to the woman whose driveway Amanda built the fire on, and she tells us, she had no idea what was going on until cops knocked on her front door. She says cops asked her, "Do you know anyone by the name of Amanda Bynes, or why she would have some vendetta against you?" She said no ... not surprising because the woman is elderly.

Cops then described to her that there was a young lady in her driveway, who had lit a fire and was carrying a little red gas tank. The homeowner says cops told her Amanda had burned part of her clothing.

Amanda Bynes is currently hospitalized on a 5150 hold ... after starting a small fire in the driveway of some random person's house Monday night, TMZ has learned.

Law enforcement sources tell us, the fire department responded to the home in Thousand Oaks, CA around 9 PM -- near where Amanda was just accused of trespassing -- after someone noticed the small blaze in the driveway. The sheriff's department was subsequently called when Amanda was found standing near the campfire.

When sheriff's deputies arrived, they questioned Amanda about what she was doing, and why she was doing it -- and based on her answers, they determined she needed to be hospitalized on a 5150 hold. Translation: her answers were really wacky.

A 5150 hold is an involuntary hospitalization for mental evaluation. Amanda can only be held for 72 hours though.

Britney Spears was famously taken in on a 5150 in 2008 during her pre-conservatorship period.

And just to give you an idea of what Amanda looked like yesterday, these photos were taken at Bloomingdales in Santa Monica just hours before the hospitalization ... around 5 PM.

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