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Demi Lovato 's one-night-only Los Angeles show might not stay that way for long ... 'cause TMZ has learned her team is actively exploring bigger venues and even a possible tour after overwhelming demand left thousands of fans in a frenzy.

Demi announced the L.A. show on her social media Monday morning, and sources with direct knowledge tell us over 200,000 people flooded the online queue for the October 25 concert at the Hollywood Palladium, which holds just 4,000 fans. Within 20 minutes, we're told the show was sold out.

Our sources say Demi's team immediately began discussing an expansion ... looking at larger venues and mapping out logistics for a potential multi-city tour tied to her upcoming album, "It's Not That Deep," which drops October 24.

We're told the show was meant

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