Protesters in Vietnam publicly denounced China’s decision to open up areas of the South China sea to bids from oil companies in demonstrations in Hanoi on Sunday, Reuters reported. The hundreds of Vietnamese protestors were responding to Chinese state-owned firm CNOOC’s (CEO.NYSE, 0883.HKG) invitation to foreign companies to explore for oil and gas in nine offshore tracts in the South China Sea, areas also claimed by Vietnam. Vietnamese authorities warned protestors not to attend rallies, although police took no action against the crowds. Petrovietnam, Vietnam’s state oil and gas group, has begun exploring in much of the same tracts in partnership with US-headquartered Exxon Mobil (XOM.NYSE, XONA.FRA), Russia’s Gazprom (GAZP.MCX) and India’s ONGC Videsh. The Vietnamese government also objected to China’s decision to give Sansha city in Hainan province administrative powers over several disputed islands in the South China Sea, seen as a move to more strongly assert China’s claim.