She said the set in front of 10,000 baying fans was the best show ever, but added she felt like she couldn't celebrate because of the incident, calling on followers to send her any footage they had.
"Tonight was beautiful. You are beautiful. But the struggle ain't over. Racism and bigotry don't care if you're a headliner," she wrote in a series of tweets and Instagram posts. "If you lay hands on my team again I hope you ready for the wrath of god."
"Thank you @summerfest. We will be filing a complaint against that bigoted 'security' guard and I hope you'll be cooperative in seeking justice."
She later added the update that the festival organizers were investigating the incident, reiterating the call for witnesses.
She added: "Friendly reminder that you don't have to say the 'n word' to be racist. That's not the sole requirement. Asking people to prove racism is another tool the oppressor uses to marginalize and discredit us."
Jennifer Lopez, Lionel Richie, Billie Eilish, Lil Wayne, Snoop Dogg, The Killers, Jason Aldean, Willie Nelson, The Roots and Steve Aoki are all scheduled to play at the ten day festival, which kicked off on Wednesday.
Summerfest told TooFab in a statement: "Lizzo gave an incredible performance which she now feels is tarnished by events which occurred during the performance. We do not tolerate racism in any form. We will conduct a thorough investigation."
"We have an experienced crowd management staff which strives to protect performers and fans. While there may be challenges during a performance, we expect those challenges to be handled professionally and respectfully. If those standards were not met, we will take appropriate action."