Vietnamese police released blogger Nguyen Ngoc Nhu Quynh on Saturday after more than a week in detention under investigation on state security grounds, her mother said by telephone from the beach town of Nha Trang.

Quynh's release comes a week after police in the capital Hanoi freed another blogger, Bui Thanh Hieu, and journalist Pham Doan Trang, who were detained in late August.

This afternoon they told me to go to the police and I saw my daughter was out already, said Quynh's mother, Nguyen Thi Tuyet Lan.

After nine days in detention they decided my daughter's actions were not serious enough so they let her free.

Quynh, who blogged under the name Me Nam or Mother Mushroom, as well as Hieu and Trang had written critically of a government plan to exploit bauxite reserves with the help of a state-owned Chinese company and of China's claims to sovereignty over islands in the South China Sea.

Hieu and Quynh were also involved in plans to print and distribute t-shirts to major Vietnamese cities with slogans against the bauxite plan and in support of Vietnam's claims to the Spratly and Paracel islands. A diplomatic source said Trang was also involved.

Vietnamese Foreign Ministry spokeswoman Nguyen Phuong Nga said on Thursday the three had been detained for investigations into possible violations of national security, although she did not elaborate. Hieu and Trang were released a week ago on the eve of a visit to Australia and New Zealand by Vietnamese Communist Party chief Nong Duc Manh.

Vietnam's relationship with China is overshadowed by a history of conflict, and the ruling Vietnamese Communist Party performs a balancing act between the need to stay on good terms with Beijing and a populace that is highly suspicious of, and increasingly vocal about, the giant neighbor to the north.

(Reporting by John Ruwitch)