It began with a minor episode. A couple from China allowed their child to relieve herself on a busy Hong Kong street.
Two Hong Kong men took exception to it and started arguing with the couple. The clip of their heated exchange caused a stir online last week.
The Hong Kong media praised the two men as heroes who had fought against ill-mannered mainland tourists.
Now the mainlanders are hitting back.
They have launched an online campaign to urge mainland Chinese parents to take their children to Hong Kong next week and let them urinate in the streets to familiarise locals with "natural functions", Hong Kong daily South China Morning Post reported.
The idea came as state media weighed in on the row, saying that both the parents and those who objected needed to "raise their level of civilisation".
User Haijiao No 68 wrote on popular mainland forum Tianya ahead of the May 1 public holiday: "Take children to Hong Kong and let them urinate in Hong Kong's streets.
"Let's see who will come and take photos. They will see it as natural after they have been familiarised with the act.
"Mainlanders are the dads and mums of Hong Kong people. Without us, can Hong Kong live carefree?"
The video images have drawn more than a million comments and reposts on Weibo alone.
It reignited the debate on social media platforms over the behaviour of mainland tourists in Hong Kong.