The mother standing next to Rochelle Mara had been laughing at the “big boy” on the rugby field, criticizing the child’s size to another parent and asking why he was being allowed to “slow the team down.”
Mara asked the woman who she was talking about. The woman pointed out her 10-year-old son, Eljae.
That afternoon, the New Zealand mom took to Facebook to post an open letter to the “parents on the sideline,” including the woman she'd called out for bullying her child.
“Ha! You couldn't move to the other side of the field fast enough. Can I just remind you, Eljae is out there training with his TEAM twice a week, he turns up every weekend to play alongside his TEAM, the same team your kid plays for!” Mara wrote in the post.
It featured a photo of her son in his No. 8 rugby jersey.
"He, nor any of the other boys/girls, need to hear you talking down about them, so please — for the love of Beyonce — stand there on that sideline along with the rest of us parents and support OUR boys and girls! ALL OF THEM. Win or lose. It's a game. They're all out there playing the best they can. Number 8 included."
Mara said she wrote the letter because she couldn't stop thinking about the woman's comments and took to Facebook "to air my frustrations."
"Basically a reminder to parents to be more cautious of what they were saying and who was around," she told TODAY.
What Mara didn’t expect was her post to go viral.
Since she wrote it on Saturday, the post has received more than 143,000 Facebook “reactions” and 17,000 comments.
It also has been shared more than 10,000 times, including by several New Zealand celebrities.
Professional rugby player Liam Messam of New Zealand's All Blacks posted Mara's letter on his Instagram account.
“We need to be there to support our kids not tear them down. #ourfuture #ledkidsbekids #igotuEljae," he wrote.
RELATED: Kelly Clarkson has 'awesome' response to body-shaming bully
Mara said support has poured in from across the world, including messages from Singapore, Canada and South Africa.
“I was absolutely blown away with people congratulating me for speaking up, offering love and support and sharing their own stories with me,” she said.
Earlier this week, Mara met to discuss what happened with her son’s coaches and managers.
They were supportive and assured her that none of the other parents would be singled out, she said.
“I didn't want them to feel like they couldn't bring their child back to the team," she said.
"That would only be taking away from the child and I want nothing more than for this kid to have the same opportunities as Eljae in the game."
RELATED: Fat-shaming cards handed out on London subway are met with these amazing responses
The team also held a parents meeting about expectations about behavior on the field.
Mara said this is her son's first time playing rugby on a team. Many of the other players are newcomers as well. The group has been training for the past eight weeks and playing in matches for the last four.
She said because her son hadn’t heard any of the initial comments, he didn’t know about the incident, or the way it had blown up over social media, until Mara showed him the Facebook post on Sunday night. She also showed him some of the many supportive comments it generated.
“He, too, was blown away and could not wipe the smile from his face knowing now that so many people were proud of him for getting out there and giving it a go!” she said.
“The support from here in New Zealand and all over the world has just been overwhelming,” she said. “Both myself and Eljae will forever be grateful for the love and support we've received over the past few days. Words don't even begin to describe.”
Follow TODAY.com writer Eun Kyung Kim on Twitter.