HONG KONG — The argument began over the seemingly minor offense of eating on the subway, which is banned in Hong Kong.
A local commuter was outraged that a girl in a tour group from mainland China was spilling noodles onto the floor.
In a video clip that has gone viral, the commuter hits an emergency stop button to alert subway staff.
"I told them they can't eat on the subway," the man tells the subway worker who answers the call. "But then they yelled at me. Hey! This is Hong Kong!"
The commuter demands an apology from the girl's mother. She refuses, and the argument explodes, with both sides screaming at each other.
The quarrel, between two groups speaking mutually unintelligible dialects of Chinese, isn't just about manners. It also illustrates how - 15 years after this former British colony was handed back to China - Hong Kongers feel less Chinese and more an island unto themselves than ever as they face a growing influx of visitors from the mainland.
And that's a headache for the communist masters in Beijing, who are concerned about the threat of disloyalty in the semiautonomous territory and are lashing out against the notion of a separate Hong Kong identity.
Other events also have highlighted the split between Hong Kongers and mainland visitors, who are frequently derided as "locusts" for their voracious buying of everything from apartments to luxury goods to baby formula.
A regular poll found the sense of Hong Kong identity surging. A flood of pregnant mainland women crossing into the city solely to give birth has strained tensions.
Even the U.S. consul general has been dragged into the debate after praising Hong Kong for sticking to the "one country, two systems" principle.
A well-known Beijing scholar turned up the heat by calling Hong Kongers "dogs" and "bastards."
Hong Kong, which returned to Chinese control in 1997 after more than 150 years as a British colony, is still a global financial center.
It's also a special administrative region with Western-style rights and freedoms not seen in mainland China, most notably freedom of speech and the rule of law.
Local council members and half the legislature are elected by voters, while a pro-Beijing committee chooses Hong Kong's leader, though direct elections have been promised as early as 2017.
Trying to control Hong Kong
Beijing's fear is that if Hong Kong continues to have an excessively vibrant culture of its own, it could prompt residents to press for a faster pace of democratization that could lead to a figure being elected as the city's leader who is not pro-Beijing.
"All this is anathema to the Chinese Communist Party," said Willy Lam, a professor at Chinese University of Hong Kong.
Mr. Lam said China's rulers have been obsessed about maintaining control in Hong Kong since July 2003, when more than half a million people took to the streets - in a massive rally that took Beijing by surprise - to protest against proposed anti-subversion legislation.
Chinese officials and pro-Beijing papers have even lashed out at the top U.S. diplomat in Hong Kong as they fret about subversion.
The papers accused U.S. Consul General Stephen Young of supporting groups trying to break Hong Kong away from China. They point to his previous posting in Kyrgyzstan in 2005 during a pro-democracy uprising there and accuse him of supporting Taiwan's independence during a posting to that island.
And a Beijing official based in the territory dismissed a Hong Kong University poll in December as "illogical" after it found that the number of people identifying themselves as being Hong Kong citizens had hit a 10-year high while those identifying themselves as Chinese citizens slumped to a 12-year low.
The attacks are signs Beijing is trying to stop Hong Kongers from clinging to their cultural, political and linguistic differences, Mr. Lam said.
"Beijing is confident that since it controls the Hong Kong economy - and the loyalty of the great majority of Hong Kong business owners - it can afford to use apparently draconian measures to suppress what it regards as the inordinate expansion of Hong Kong identity," he said.
Those measures include attempts to introduce Chinese patriotism lessons in schools, preventing dissidents from entering the city and using Hong Kong's pro-Beijing press to attack people or groups it deems unfriendly.
While Hong Kongers and mainlanders are both ethnically Chinese, people in the city on the southern edge of China tend to look down on their mainland brethren as uncouth and uncultured.
What's new is that many of the mainlanders flooding into Hong Kong nowadays are now much wealthier - and their influence much greater - thanks to the country's emergence as the world's second-biggest economy.
Italian fashion house Dolce & Gabbana stirred local anger after reports it was banning Hong Kongers from taking photos of one of its boutiques while allowing wealthy mainland shoppers to snap away. The fashion house later apologized.
"I don't have a problem with mainlanders per se, but in the past year they've been coming to Hong Kong to give birth and then leaving without paying their fees. They're taking our resources, but they're not paying Hong Kong back," griped Sun Wong, an 18-year-old student. "It offends me."
Nearly 33,500 children were born in Hong Kong last year to parents who both live on the mainland, up from 620 in 2001. Many are trying to escape China's one-child policy, but Hong Kongers complain they also take up hospital beds that should go to locals.