Vietnam police have launched a formal criminal investigation into an American man who was among dozens arrested last week during rare protests that erupted across the authoritarian state.
American citizen William Anh Nguyen, 32, was detained in Ho Chi Minh City on Sunday during a massive rally protesting a government proposal to grant lengthy land leases in new special economic zones.
Many protesters expressed fear the land would be handed to China, and thousands gathered in several cities across the country to demand the government not lease land to Beijing for "even one day".
At least 40 people are still in detention after the rallies, according to an AFP tally using official figures and state media reports.
The protests turned violent in several spots including southern Binh Thuan province where demonstrators torched police buses and damaged government property.
Ho Chi Minh City police said late Friday that Nguyen was being investigated "for acts of disturbing public order" and accused him of inciting protesters and trying to damage public property.
"After seeing people trying to flip a police pickup truck to clear the way for the protesters, William Anh tried to help them flip the vehicles," said a report from Ho Chi Minh City's official police newspaper.
The investigation could lead to formal charges and means Nguyen is likely to remain in detention until the probe is closed.
The Texas-born Yale graduate was passing through Vietnam on holiday before going to Singapore where he was set to receive a Masters degree next month.
Nguyen's friends say he was injured before his arrest, citing footage on social media that shows him with a bloody wound on his head as he is dragged to a vehicle by plainclothes men.
He was visited by US consular officials in jail on Friday and appeared "in decent spirits", family friend Kevin Webb told AFP.
"He seemed to be in decent health, we don't know if he's received medical attention for his head injury," said Webb, who has been working with Nguyen's family to lobby lawmakers in Washington for his release.
A US state department spokesman told AFP that they are concerned by reports that Nguyen was injured and that American officials are pushing for continued consular access to Nguyen.
Family friends said he did not bring a political agenda to protests but was only there to celebrate the right to free assembly.
"He was not at these protests to express any kind of political message," Mary Daniel told AFP.
Authorities distributed leaflets and sent text messages in Ho Chi Minh City this week warning residents not to join illegal gatherings and there was a heavy police presence in the city on Saturday.
The draft law on special economic zones made no mention of granting land to China, but ignited a long-simmering resentment against Vietnam's powerhouse communist neighbour.
Hanoi and Beijing have a historically checkered relationship since a bloody border war in 1979, and tensions frequently spike over disputed territory in the South China Sea.