A Chinese cruise ship with more than 300 passengers visited Vietnam's Hoang Sa (Paracel) Archipelago earlier this month.
"Vietnam strongly opposes this and demands that China respect Vietnam's sovereignty over the Paracel Islands and international law and immediately stop and not repeat those activities," foreign ministry spokesperson Le Hai Binh told Reuters.
"Those actions have seriously violated Vietnam's sovereignty over the Paracel Islands and international law."
China claims 90 percent of the potentially energy-rich East Vietnam Sea. Brunei, Malaysia, the Philippines, Vietnam and Taiwan lay claim to parts of the sea, through which passes about $5 trillion of trade a year.
On Monday, Chinese foreign ministry spokeswomen Hua Chunying told a regular news briefing that she had not seen the comments by Vietnam, but reiterated China's claim to the Paracel Islands.
Countries competing to cement their rival claims have encouraged a growing civilian presence on disputed islands in the East Vietnam Sea. The first cruises from China to the Paracel islands were launched by Hainan Strait Shipping Co in 2013.