The foreign ministry sent a note to its Chinese counterpart Tuesday protesting that country's illegal establishment of the so-called Sansha city to administer Vietnam's Truong Sa (Spratly) and Hoang Sa (Paracel) archipelagos.
In the note, the Ministry of Foreign Affairs, said this "seriously" violates international laws and Vietnam's sovereignty over the islands, Vietnam News Agency reported quoting Luong Thanh Nghi, the ministry spokesman.
It said the city's establishment and related activities are against the ground rules for resolving maritime issues the countries agreed to last October and the Declaration of Conduct (DOC) signed between ASEAN and China in 2002.
The acts also complicate the East Sea situation, the news agency quoted him as saying.
Vietnam objects to the acts and demands that China should immediately stop all the wrongful actions, respect Vietnam's sovereignty, and make practical contributions to the countries' friendship and to peace and stability in the East Sea, the note said.
The same day authorities in the central city of Da Nang and the central province of Khanh Hoa issued a joint announcement protesting China's actions.
"Da Nang city and Khanh Hoa Province strongly object to (the acts), and demand that China immediately stops all actions that violate international laws and Vietnamese laws, hinder efforts to maintain peace and stability in the East Sea, hurting peoples' friendship," they said.
"China's wrongful acts do not change the fact that Hoang Sa and Truong Archipelagos are inseparable parts of Vietnam's territory."
The protests follow a decision by China's Central Military Commission to establish a Sansha city military command with a base on Phu Lam Island in Hoang Sa.
This was on July 19, and two days later China organized elections to the city legislature despite objections from Vietnam and international experts.
It recently organized a ceremony to mark the establishment of Sansha on Phu Lam attended by senior officials from the transport, civil affairs and public security ministries, Xinhua news agency reported Tuesday.
The ceremony was broadcast live on China Central Television.
Earlier, on June 23, authorities in Da Nang and Khanh Hoa had made similar protests after China announced its decision to create Sansha.