HANOI, Vietnam — A Vietnamese-born American citizen who is a member of a pro-democracy group has been detained in Ho Chi Minh City, a family member and an attorney said Monday.

Cong Thanh Do, 47, of San Jose, Calif., was taken into custody August 14 in the central Vietnamese city of Phan Thiet and later transported to Ho Chi Minh City, his daughter, Bien Dobui, said by telephone from California.

Mr. Do, who is a member of the People’s Democratic Party of Vietnam, an anti-communist group that wants a multiparty system in Vietnam, was accused of being involved in terrorist activities, Ms. Dobui said.

However, defense attorney Nguyen Van Dai, who has been asked to represent Mr. Do, said the reason for his detention remains unclear. Mr. Dai is expected to meet with Mr. Do for the first time in Ho Chi Minh City later this week.

The U.S. Embassy in Hanoi confirmed that Mr. Do was arrested and has met with officials from the U.S. Consulate in Ho Chi Minh City. Ms. Dobui said her father has begun a hunger strike that would “end either in his freedom or his death.”

Mr. Do, an engineer in Sunnyvale, Calif., immigrated to America in 1982 and has returned to Vietnam several times since, Ms. Dobui said. He was on vacation in Vietnam with his wife and son when he was detained, she said.