THE pictures spread like wildfire across the Internet. They show Mr Nguyen Van Phuong being grabbed by muscular black-shirted police at an anti-China protest in Hanoi on Sunday.
A policeman has one arm in a lock around Mr Phuong's neck; with the other, he clamps his hand over the protester's face as he is lifted and taken away in the middle of a knot of struggling people. He was put in a mini-van. Ten protesters lay on the road to block the van but the police managed to take him away, releasing him a few hours later.
On his Facebook page later, Mr Phuong, who described himself as a marketing manager, posted: "I just want to ask them (the government): Why, when our territory is violated and people are killed at sea, are you ignoring it? Why, when we express our anger against the Chinese enemy in a peaceful way, you consider us the enemy?"
Vietnam, while speaking out more in international forums, is navigating choppy waters at home, with the one-party state caught between a more demanding citizenry, differences within the party and anti-Chinese sentiment over the South China Sea. Managing the confluence of domestic and regional tensions, analysts said, is a major challenge.
As citizens become more outspoken - mostly online to get around the sanitised local media - the Communist state has responded in recent months with a wave of repression. Last September, three bloggers were jailed for "anti-state propaganda". Dozens more have been jailed, some for allegedly plotting to overthrow the government. Thirty-three were reportedly jailed last year, and so far this year, 36 have been jailed.
Dissatisfaction - some of it with the slowing economy - has surfaced in recent months.
Earlier this year, Mr Le Hieu Dang, a former vice-chairman of a party-run organisation in Ho Chi Minh City - and a 40-year member of the ruling Communist Party - embarrassed the government by saying the party had forgotten its origins and was "going against the rights of the people".
A new rallying point is China's claim to the entire South China Sea - disputed by other countries, including Vietnam and the Philippines. Vietnam even has its own name for it, calling it the East Sea.
Police often outnumber protesters and have hauled them away in buses but usually released them after a few hours. But inevitably, said observers, sentiment at the protests is morphing from purely anti-China, to anti-police and anti-state.
"Has there ever been a period in Vietnamese history as disgraceful as this?" Mr Phuong wrote on his Facebook page after he emerged from detention.
In March, a Chinese naval ship fired flares at Vietnamese vessels fishing near the group of islands known as the Paracels and set one ablaze. Such incidents have heightened fears of an accidental escalation in the disputed sea.
Two days before Sunday's protests, Vietnamese Prime Minister Nguyen Tan Dung, speaking at a security forum in Singapore, without naming China, said the South China Sea disputes threatened regional security. He said: "Somewhere in the region, there have emerged preferences for unilateral might, groundless claims and actions that run counter to international law and stem from imposition and power-politics."
But Vietnam, while anxious not to be seen as kowtowing to China, also cannot afford to let anti-China sentiment gain too much traction. China is a large and powerful neighbour and Vietnam's largest trading partner. Vietnam is also "generally keen not to lose control of political expression," said City University of Hong Kong professor Jonathan London.