The United States on Tuesday called for the release of peaceful protesters detained when Vietnamese security forces broke up a weekend rally against Beijing's actions in the South China Sea.

"We are concerned by the detention of several individuals for what appears to be the peaceful expression of their views," a spokesman for the US embassy told AFP.

"We call on the Vietnamese government to release all individuals detained for exercising their human rights and fundamental freedoms."

An official police newspaper reported on Monday that 47 people were initially detained at Sunday's rally in central Hanoi.

Protesters objected to China's "invasion" of waters where the two nations have a longstanding sovereignty dispute.

Thirty-nine were released but another eight were being investigated for public disorder and resisting officials performing their duties, said the official police newspaper, An Ninh Thu Do.

The embassy spokesman, who declined to be named, said nobody should be detained for exercising the right to peaceful assembly.

"This contradicts Vietnam's obligations under the International Covenant on Civil and Political Rights," he said.

Vietnam says it has achieved significant progress on human rights.

Five more detainees were released late Monday, said Nguyen Quang Thach, 36, one of those freed.

Under the Vietnamese legal system, the other three could be held until Wednesday for initial investigation.

"The police have been very polite and friendly. We worked in terms of cooperation, not investigation," said Thach. He vowed not to demonstrate any more but to continue his anti-China struggle in other ways.

Sunday's rally over maritime tensions was the 11th since early June, an unprecedented run in an authoritarian communist country where overtly political demonstrations are rare.

Although people were briefly detained after two earlier demonstrations, subsequent protests were allowed to go ahead until Hanoi authorities on Thursday finally clamped down and issued a stop order.

Prominent intellectuals linked to the protests have denied official allegations that "anti-state forces" took advantage of the demonstrations.

Nguyen Xuan Dien, a scholar whose blog ( became a rallying point for the protests, wrote that he was to be questioned by police investigators on Tuesday.