Spencer Pratt doesn't think Justin Bieber's "impromptu" park performance is as off the cuff as Los Angeles County District Attorney Nathan Hochman is making it seem ... he says he's maddened by BS "propaganda."

The reality TV star -- who came up short in L.A.'s mayoral primary -- wrote on social media Wednesday ... "Yes, an 'impromptu' set with generator-powered amps and blacked out armed security SUV directly behind him."

Spencer was responding to a Hochman's post about Bieber playing a set at MacArthur Park near downtown Los Angeles ... with Hochman saying the "impromptu" performance was "a testament to the progress we are achieving to take back the park from drug dealers."

In his response, Spencer fumes ... "This bulls*** propaganda is maddening. Stop pissing on our legs and telling us it's raining. Send your kids to play barefoot soccer there without security."

As we previously reported, sources familiar with the situation said the "purpose" of JB's visit was to help the homeless ... he played music and dropped thousands on gift cards, socks, water, and supplies and passed them out to those in need.

Spencer sees it differently, though.