Authorities in Ho Chi Minh City would not comment on the reported arrests Saturday of the four foreign activists plus two Vietnamese citizens.
The overseas-Vietnamese group Viet Tan (Reform) said the activists, all members of the pro-democracy organisation, were meeting with Vietnamese citizens to discuss "peaceful democratic change" in Vietnam.
Police surrounded the house where the meeting was taking place and then raided the home with more than a dozen officers, seizing materials, Viet Tan said citing a witness to the raid.
Among other things, the activists were passing out copies of a book called From Dictatorship to Democracy in a Vietnamese translation, according to a Viet Tan spokeswoman.
Communist-run Vietnam bans any political opposition and "propaganda against the Socialist Republic" is a crime that carries prison terms of up to 20 years.
In the past year, about a dozen prominent Vietnamese dissidents have been arrested and sentenced to lengthy jail terms, often accused by authorities of colluding with "hostile forces" based overseas.
Viet Tan named its arrested Vietnamese-American members as Nguyen Quoc Quan of California and Leon Truong of Hawaii. Both are US citizens born in Vietnam, the group said.
French-Vietnamese writer Nguyen Thi Thanh Van and Thai national Khunmi Somsak, who is ethnic Vietnamese, were also picked up in the raid by more than a dozen police on a house in Ho Chi Minh City, the group said.
The French and US embassies in Hanoi said Tuesday they had not been informed by the Vietnamese government of the arrests but were looking into the matter.
"We're trying to get consular access now," said US Embassy spokeswoman Angela Aggeler.
She said that the Vietnamese government, when contacted by the embassy following a Viet Tan statement on the arrests, confirmed the arrest of only one US citizen and did not reveal what he was being accused of.
Vietnamese police contacted Tuesday refused to discuss the arrests.
"We are not permitted to talk about this," said Nguyen Dinh Minh, police chief of the Ho Chi Minh City neighbourhood where the arrests reportedly happened.
Vietnam has in recent years arrested several US citizens working for an end to the Communist Party's monopoly on power.
Vietnamese-American essayist Cong Thanh Do was originally accused of plotting to bomb the US consulate in Ho Chi Minh City, though the US Embassy said there was no evidence of such a plot.
Do's family insisted he was being targeted for his pro-democracy activism. He was eventually released without charge and deported.
Three other Vietnamese-Americans were arrested with radio equipment and charged with plotting to override national state-run radio stations with their own broadcasts.
They were held for more than a year before being tried and convicted on terrorism charges but released and deported days later.//dpa