Roll Call said the rapper performed "Words I Never Said" over and over for more than 30 minutes Sunday night, prompting some audience members to boo and shout at him or leave the venue in protest.
His song said the "war on terror" is "a bunch of [expletive]. Just a poor excuse for you to use up all your bullets."
The lyrics talk about "coverups" and budget cuts that hurt children.
Fiasco said if you turn on the TV "all you see's a buncha ... dude is dating so and so, blabbering 'bout such and such and that ain't the Jersey Shore, homey, that's the news."
His song calls talk show hosts Rush Limbaugh and Glenn Beck racists and says the "Gaza Strip was getting bombed -- Obama didn't say [expletive]. That's why I ain't vote for him, next one either. I'm part of the problem, my problem is I'm peaceful.And I believe in the people."
The organizers of the StartUp RockOn event issued a statement to Roll Call saying Fiasco left the stage "earlier than we had planned," but was not "kicked off stage" for an "anti-Obama rant," as some media reports suggested.
"We are staunch supporters of free speech and free political speech. This was not about his opinions. Instead, after a bizarrely repetitive, jarring performance that left the crowd vocally dissatisfied, organizers decided to move on to the next act," the statement said.
Roll Call cited unnamed sources as saying StartUp RockOn has stopped payment on Fiasco's paycheck for the show.