* Vietnam spokesman calls on China to stop "wrongful acts"
* Petrovietnam says seismic cable cut, accuses Chinese boats of sabotage
* India says it will protect its maritime and economic interests
* South China Sea tensions escalate as China flexes muscles
HANOI/NEW DELHI, Dec 4 (Reuters) - Vietnam condemned on Tuesday China's claims to disputed South China Sea islands as a serious violation of its sovereignty after saying it was setting up patrols to protect its fisheries and accusing Chinese boats of sabotage.
The condemnation of China's claims to the sea and its numerous reefs and tiny islands was the strongest yet from Vietnam since tension flared this year and came after India declared itself ready to send navy ships to safeguard its interests in the disputed waters.
Claims by an increasingly powerful China over most of the South China Sea have set it directly against U.S. allies Vietnam and the Philippines, while Brunei, Taiwan and Malaysia also claim parts of the mineral-rich waters.
Vietnam's condemnation came a day after its state oil and gas company, Petrovietnam, accused Chinese boats of sabotaging an exploration operation by cutting a seismic cable being towed behind a Vietnamese boat.
Vietnam's Foreign Ministry spokesman condemned the cable cutting as well as some recent Chinese provincial regulations that identified the disputed Spratly and Paracel islands as Chinese, and a map that did the same thing.
"The actions of the Chinese side have seriously violated Vietnam's sovereignty over the two archipelagos," the spokesman, Luong Thanh Nghi, said in a statement.
Vietnamese Foreign Ministry officials met representatives of the Chinese embassy in Hanoi on Monday, Nghi said.
The Vietnamese officials handed over a diplomatic note "resolutely opposing the above mentioned actions by the Chinese side, asking China to respect Vietnam's sovereignty, to immediately stop such wrongful acts and not to repeat similar actions."
Earlier, Vietnam said civilian-led patrols, backed by marine police and a border force, would be deployed from Jan. 25 to stop foreign vessels violating fishing laws in Vietnam's waters.
A decree on the Vietnamese patrols was signed on Nov. 29, the day Chinese media announced new rules authorising police in the southern Chinese province of Hainan to board and seize foreign ships in the South China Sea.
"It's going to lead to friction," Carl Thayer, a Southeast Asia security expert at the University of New South Wales in Australia, said of China's new rules that take effect from Jan. 1 on boarding ships which "illegally enter" waters it claims.
"If it begins to assert these rights and isn't challenged, over time it becomes customary, it becomes practice."
On Monday, Petrovietnam said the seismic vessel had been operating outside the Gulf of Tonkin when the cable was severed on Friday. It had earlier been surveying the Nam Con Son basin further south - an area where Indian state-run explorer Oil and Natural Gas Corp (ONGC) has a stake in a Vietnamese gas field.
Petrovietnam posted on its website comments by the deputy head of exploration, Pham Viet Dung, in which he said the cable was repaired and the survey resumed the following day.
"The blatant violation of Vietnamese waters by Chinese fishing vessels not only violates the sovereignty ... of Vietnam but also interferes in the normal operations of Vietnamese fishermen and affects the maritime activities of Petrovietnam," Dung was quoted as saying.
Asked about the complaint, Chinese Foreign Ministry spokesman Hong Lei told a briefing in Beijing that China was checking the reports of the incident, which he said was understood to have taken place in an area of overlapping claims.
"Chinese fishing boats were operating in normal fishing activities," Hong said.
India has also declared itself ready to deploy naval vessels to the South China Sea to protect its oil-exploration interests there, a new source of tension in a disputed area where fears of conflict have been growing steadily.
Indian navy chief, Admiral D.K Joshi, said on Monday that, while India was not a territorial claimant in the South China Sea, it was prepared to act, if necessary, to protect its maritime and economic interests in the region.
"When the requirement is there, for example, in situations where our country's interests are involved, for example ONGC ... we will be required to go there and we are prepared for that," Joshi told a news conference.
"Now, are we preparing for it? Are we having exercises of that nature? The short answer is yes," he said.
An Indian government spokesman on Tuesday played down the comments: "This is an issue for the parties concerned to resolve."
India is not the only non-claimant nation concerned about disruption to shipping or oil exploration in the South China Sea. The United States, a close ally to several of the Southeast Asian claimants, has also voiced concern at the prospect of China stopping international ships in contested waters.
India has sparred diplomatically with China in the past over its gas and oil exploration block off the coast of Vietnam.
Any display of naval assertiveness by India in the South China Sea would likely fuel concern that the navies of the two rapidly growing Asian giants could be on a collision course as they seek to protect trade routes and lock in the supply of coal, minerals and other raw material from foreign sources.
Joshi described the modernisation of China's navy as "truly impressive" and a source of major concern for India.
Asked what China would do if Indian navy entered the South China Sea to protect its oil interests, the Chinese Foreign Ministry spokesman, Hong, said China had "indisputable sovereignty" over the sea's islands and surrounding waters.
"China opposes unilateral oil and gas development in disputed waters of the South China Sea. We hope that concerned countries respect China's position and rights, and respect efforts made through bilateral talks to resolve disputes."
Singapore, home to the world's second-busiest container port, joined the Philippines on Monday in expressing concern at the prospect of Chinese police boarding ships. The Philippines on Saturday condemned the Chinese plan as illegal.
Estimates for proven and undiscovered oil reserves in the South China Sea range as high as 213 billion barrels of oil, the U.S. Energy Information Administration said in a 2008 report. That would surpass the proven oil reserves of every country except Saudi Arabia and Venezuela, according to the BP Statistical Review.
On Monday, China's National Energy Administration said China aims to produce 15 billion cubic metres of natural gas a year from the South China Sea by 2015.
It said the sea would "form the main part" of China's offshore gas exploration plans.