Eight anti-China demonstrators who defied government orders to end an unprecedented series of rallies remain in custody for investigation, an official Vietnamese police newspaper reported on Monday.

They were among 47 detained at the rally on Sunday beside Hoan Kiem lake in central Hanoi, An Ninh Thu Do reported.

Protesters were objecting to China's "invasion" of South China Sea waters where the two nations have a longstanding sovereignty dispute.

Thirty-nine were later released but the other eight were being investigated for causing public disorder and resisting officials performing their duties, the paper said. Both crimes can carry jail terms.

Under the Vietnamese legal system, suspects can be held for an initial 72 hours for investigation.

Police contacted by AFP on Monday said they had no information about the case.

Attempts to reach suspected detainees by their mobile phones have proven unsuccessful since Sunday.

Le Quoc Quan, a lawyer in touch with relatives of some of those held, told AFP: "There are at least eight people detained at Hoan Kiem police station."

He added that he hoped they would be released Monday evening.

Minutes after the protest began at the Hanoi lake, which is popular with tourists and locals, plainclothes agents moved in to force the demonstrators onto two waiting buses and drove them away.

Sunday's rally was the 11th since early June to protest Chinese actions in the tense South China Sea. Two earlier demonstrations were dispersed by police but those detained were released the same day.

Subsequent protests were allowed to go ahead until authorities in the Vietnamese capital on Thursday issued a stop order.

"The first gatherings, demonstrations and rallies came from the patriotism and the anger of the people," said An Ninh Thu Do, the official Hanoi police newspaper.

However, more recent demonstrations were instigated by "anti-state forces in and outside the country," it said. Prominent intellectuals linked to the peaceful protests have denied such allegations.

Overtly political demonstrations are rare in authoritarian Communist Vietnam, but analysts said authorities had previously allowed the anti-China rallies because they helped express Hanoi's displeasure with Beijing over the maritime tensions.

The government also has to balance its relationship with China, analysts say, by not overtly offending its giant communist neighbour while avoiding the appearance of weakness before its own people, who bitterly resent 1,000 years of Chinese occupation.