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 9pm -- 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 5pm.

Story developing ...

Amanda Bynes didn't just build a fire in a driveway Monday night ... she put a flaming cloth on top of a gas tank ... effectively creating a makeshift explosive -- this according to an eyewitness 911 call.

The caller describes it as a small fire -- a flaming cloth sitting on top of a gas tank in a residential driveway. He says the reason he called 911 is because of the gas tank ... i.e. the obvious possibility of a massive explosion.

The caller says he couldn't tell if the gas tank was full or not.

Thankfully, the gas tank did NOT explode -- but as we reported, the fire department rushed to the scene to extinguish the fire ... and sheriff's deputies placed Bynes on a 5150 psychiatric hold.

Cops confronted Amanda Bynes outside a retirement community after she got into an argument with a cab driver Sunday night ... TMZ has learned.

According to the cab driver ... Amanda was trying to get into an old folks home in Thousand Oaks -- outside L.A. -- when management at the place turned her away because they felt she was drunk and accused her of trespassing.

However, the retirement community was nice enough to call a taxi for Amanda -- but when she got into the cab she said ... "Get me the f**k out of here ... I don't have any money." Naturally, the driver kicked her out at that point.

We're told Amanda walked back toward the retirement community ... and that's when cops were called.

There's no record of Amanda getting arrested ... so, we're guessing she might have only been ticketed, possibly for trespassing.

Amanda grew up in Thousand Oaks -- and owns a house in nearby Calabasas.

The cab driver tells TMZ ... he was told Amanda was at the retirement village to visit a relative.