Breastfeeding Mama Talk took to Facebook to share a photo of the sign, which read: "If you need to breastfeed, please cover yourself" alongside a couple of photos of their members.
The mums took the photo literally, putting shirts and blankets over their heads while their babies were being fed.
A member of the support group wrote: "We wanted to do something different and fun with this", and asked their followers to start the hashtag #ThisIsHowWeCoverBFMT.
Many came forward with their own photos showing them finding inventive ways to "cover themselves".
A wellbeing site, The Pregnancy Corner, also joined in and uploaded their own version asking fans to help it go viral.
The post received more than a million likes, 385,000 shares and thousands of comments of support.
One woman said: "What is so wrong about it? When YOU See girls half naked on the street with their boobs and butt out twerking, people cheer. But when a mother is feeding her hungry child it is a bad thing."
This isn't the first time breastfeeding mothers have spoken up after being asked to the "cover up". One woman was told to do the exact same on a flight earlier this year.
Gemma Leung was preparing to breastfeed her eight-month-old daughter Ruby-Bow on an EasyJet flight when a male flight attendant reportedly asked her to "put a blanket over [her] baby" because it "might offend other passengers."
Leung said she "couldn't speak" because she was so angry.
Confrontation over breastfeeding in public frequently comes up in the news, including a recent incident in which a man in a cafe told a woman to stop breastfeeding, before receiving his comeuppance.
Increasingly mums are asking when will breastfeeding become something that people don't feel the need to talk about all the time?
One HuffPost UK blogger, Jo Eden, said people need to stop making a fuss over breastfeeding: "Nobody needs to normalise a normal thing. We need to stop making such a fuss over normal things. Stop abusing people on Facebook. Stop causing a scene in public."