Nicole O'Connor, a nurse practitioner in St. Louis, was selling her dining room table on Facebook when a prospective buyer — a black man — reached out. She told her husband not to leave her alone if the man came to see the furniture, which she didn't feel the need to say when a white woman like herself had shown interest.

That was her moment of clarity: "I was like, 'Oh my God, that's racism,'" O'Connor, 32, said in a group of all white adults this month. "So I told my husband. I'm like, 'I don't know what to do with this, but this is how I'm feeling.'"

O'Connor felt vulnerable and embarrassed but also understood as she shared the story with seven others sitting in a circle at a school in suburban St. Louis. No one passed judgment. This was their "white space" — a concept that has been growing in communities like St. Louis where racial incidents have prompted anger and even unrest.

A stream of viral videos this year involving white people — them calling police on black people doing ordinary activities perceived as suspicious, threatening to call ICE on Spanish-speaking workers or in racist rants — resonated in ways that have frustrated and disturbed not only minorities but white Americans who want to make sense of what they're watching.

An NBC News|SurveyMonkey poll released in May found that 64 percent of respondents believe racism remains a "major problem" in America, and while 40 percent of blacks said they were treated unfairly in a store or restaurant, only 7 percent of whites said the same.

O'Connor's group is one of several that began through the Metro St. Louis chapter of the YWCA, which has focused on racial justice in a region that has been historically segregated and where blacks have faced higher rates of poverty, infant mortality and unemployment than whites. The YWCA's program — based on the 2010 book "Witnessing Whiteness" — began in 2011, but interest climbed after the fatal shooting of black teenager Michael Brown in the suburb of Ferguson in 2014, said Mary Ferguson, the chapter's racial justice director.

Now, as many as 16 groups meet in schools, churches and other community spaces, with up to 25 people in each, Ferguson said. More than a dozen more groups are set to begin meeting in January.

Enrollment is free and the groups are organized by volunteers, but there remains one catch: Participants must identify as white.

"It was important to us that we had a group where people of color wouldn't be on the spot, wouldn't be asked to teach, wouldn't be asked to listen to white people as they struggle to understand racism," she added.

Ferguson said having a place for white people to meet also promotes a more candid conversation. The sessions focus on book chapters such as "Culture, Tradition, and Appropriation?" and "Positions of Privilege."

"One of the greatest fears that many of our participants express ... is the fear that they're going to offend," Ferguson said. "That they are going to show their ignorance, that they are going to upset other people and they sense it themselves. One way that we can open up the space for conversation is to make the group all white."

Such "whiteness" programs have been scrutinized by whites and people of color who believe the courses are their own form of segregation.

The University of Colorado at Colorado Springs scrapped an "Unmasking Whiteness" class in May since it was not intended for all races, The Gazette, the local newspaper, reported.

But the assumption that America has moved into a "post-racial" space — particularly after the election of Barack Obama as president in 2008 — is a fallacy, said Debby Irving, who is white and the author of "Waking Up White and Finding Myself in the Story of Race," about the concept of white privilege.

This year's trend of white women, in particular, calling police on black people — including a man trying to enter his own apartment building in St. Louis and a young boy in Brooklyn, New York, wrongly accused of sexual assault — shows that "white ignorance and white silence" remain chronic problems in America, Irving said.

O'Connor, the nurse practitioner, said she had never heard the term "white privilege" until this year, but through "Witnessing Whiteness" sessions has realized how racism can be subtle and unintentional, not as overt as waving a Confederate flag and shouting the N-word, but real nonetheless.

Nicole O'Connor said she had never heard the term "white privilege" until this year. NBC News

One chapter in the book, she told the group, made her think, "Wow, I'm racist," and that she "used to think I was this perfect little white person in a bubble that didn't do anything bad to black people, and so I was OK."

Speaking with her husband after the furniture-selling incident made her see the progress she was making because she could at least recognize and vocalize her subtle racism.

"This class has done a lot for me in just the awareness and understanding of where this exists in my life," O'Connor told the group. "Do I know what to do with it right now? No. But you've got to start somewhere."

Other people in the group were working out their own experiences: One man said the feeling of white guilt alone wasn't going to stop the shootings of black men by police.

A woman wondered how she could be labeled as "privileged" if she was lower-middle class and worked hard just to get there.

Vincent C. Flewellen, who is African-American and the former director of equity and inclusion at a St. Louis-area private school that offered a "Witnessing Whiteness" program, said going through the sessions doesn't automatically transform someone. It's a process.

But at a minimum, he said, he wants white people to not call police on black people "just because they're gathering in a park." Flewellen is the chief diversity officer for another nearby school, Webster University, which plans to introduce the program in 2019.

He hopes people who participate "find their voice and are able to speak to, call out and stand up against racism."

Sue Dersch, a group leader, has been trying to do just that. She began as a participant in the program in 2014, after the shooting in Ferguson. She hadn't necessarily thought of herself as racist — in fact, she and her husband had adopted two black sons more than 10 years ago.

But going through the program led her to re-examine her life experiences. One time, before the adoption, she said she tensed up and became fearful when she saw a young black man in a hoodie walking toward her in a parking lot. A year ago, while she was stuffing letters into a mailbox, a young black man was walking toward her — again, she said, she grew tense.

Dersch — no stranger to being targeted when her "Black Lives Matter" signs were vandalized repeatedly outside of her home three years ago — was mortified that she had had the same reaction. "I had the overwhelming feeling of, 'Oh my goodness, some other women could be feeling that same way about my sons,'" said Dersch, 63. "That really kind of shook me."

When thinking about videos of white women calling police on black people — garnering nicknames on social media like "Pool Patrol Paula" and "Barbecue Becky" — she sometimes wishes she could freeze-frame the situation and tell them: "It's not your business. ... Take a step back, stay in your lane, go get your groceries, and go home and fix dinner."

Ultimately, she said, it's easy to be critical of others, but white people can't be afraid to turn the mirror on themselves for fear of being branded as racist. For her, it's a progression, peeling back layer after layer of built-in bias.

Her goal? "I want to be less racist tomorrow than I am today," Dersch said.