SINGAPORE - An American student detained in Vietnam for involvement in anti-government protests, has expressed regret for breaking the law and promised to stay away from future rallies.

William Nguyen, 32, appeared on Vietnamese state television on Monday saying: "I understand that my acts violated (the law)... I regret that I caused trouble for people heading to the airport. I blocked traffic and caused trouble to my family and friends."

"I will not join any anti-state activities any more," reported Agence France-Presse quoting him.

The graduate student of the Lee Kuan Yew School of Public Policy (LKYSPP) was detained in Ho Chi Minh City on June 10 after attending mass demonstrations against draft laws to develop special economic zones which would grant investors 99-year leases.

Protesters feared that this would allow valuable land to end up in the hands of China, with which Vietnam has a fraught relationship since a deadly border war in 1979.

Vietnam police had said he was being investigated for "causing public disorder", and accused him of trying to damage a fence and flip over police vehicles on the main road to the airport, said AFP on Tuesday.

But a close friend of Nguyen's, Ms Inkar Aitkuzhina, 24, said in his defence that it seemed he was "in the wrong place at the wrong time".

"He really loves Vietnam, and I don't believe he went there with the intent to do anything wrong or violent," she said of Nguyen, who is a US citizen born and raised in Texas.

He was there on a vacation ahead of his graduation from LKYSPP in July.

Another friend, lawyer Azira Aziz, 32, also a student at LKYSPP, said: He cares a lot about South-east Asian politics, and the state of refugees."

On the day of the protest, Nguyen tweeted photos of the crowds and wrote on his Twitter account: "This is #democracy in #Vietnam". He also posted that police had struck a protester and that chaos had broken out.

Video footage circulating online later showed Nguyen himself with blood on his head, being dragged by men in plainclothes through the streets, before being taken into custody.

Nguyen's sister Victoria, who is based in the US, told The Straits Times she did not know when her brother might be released.

"We're doing the best we can," she said. "I'm angry, I'm frustrated at his arrest... It's a technique to instil fear in people."

Speaking to The Straits Times, Human Rights Watch Asia division deputy director Phil Robertson said it was likely Nguyen would be released following his televised confession.

"Given that he has given a confession that was likely extracted under duress, I expect that Vietnam will likely release and deport him, and unfortunately blacklist him from ever entering the country again," he said.

He added that Vietnam does respond to external pressure, noting that Nguyen's case has caught the the attention of the United States Congress.

Last week, three US Congressmen spoke with US Ambassador to Vietnam Dan Kritenbrink to "express their serious concerns about the arrest and detention".

Ms Nguyen on Tuesday had posted pictures of a letter signed by 15 members of Congress calling for US Secretary of State Mike Pompeo to make "swift intervention" on the case.

A petition on Change.org titled "Release All Demonstrators for Protest of Special Economic Zones and Cyber Security Bill" and calling for Mr Nguyen's release received 18,650 signatures as of Tuesday.

In response to media queries, an LKYSPP spokesman said the school is aware of the incident and is co-operating with the authorities on this matter.