China has accused Vietnam of "gravely violating" its sovereignty in an escalating row over disputed waters in the South China Sea.
Beijing said Vietnam had endangered Chinese sailors' lives and warned it to stop "all invasive activities".
It follows an accusation by Vietnam that a Chinese fishing boat rammed cables from an oil exploration vessel inside its exclusive economic zone.
China is engaged in maritime border disputes with several countries.
The South China Sea includes important shipping routes and may contain rich oil and gas deposits.
The Philippines, Malaysia, Brunei and Taiwan also have rival claims in the area; China's claim is by far the largest.
The US has also expressed concern about China's rising naval ambitions.
China's foreign ministry spokesman Hong Lei said Chinese fishing boats were chased away by armed Vietnamese ships on Thursday.
He said that during the incident the fishing net of one of the Chinese boats became tangled with the cables of a Vietnamese oil exploring vessel which continued to drag the Chinese vessel for more than an hour before the net had to be cut.
China insists the Vietnamese vessel was operating illegally in the area.
"By conducting unlawful oil and gas surveys in seas around the Wanan Bank of the Spratly archipelago and by driving out a Chinese fishing vessel, Vietnam has gravely violated China's sovereignty and maritime rights," said Mr Hong.
"China demands that Vietnam cease all violations," he said, adding that Vietnam should "not take actions that would complicate and expand the dispute".
Beijing's strong-worded statement followed Vietnam's accusation that a Chinese fishing boat had "intentionally rammed" the exploration cables of a Vietnamese boat - the second such incident in two weeks.
That vessel, chartered by state energy giant PetroVietnam, was conducting a seismic survey inside its 200 nautical mile exclusive economic zone on Thursday, said foreign ministry spokeswoman Nguyen Phuong Nga.
She described the "premeditated and carefully calculated" attack as part of China's attempts to control disputed waters.
"This is unacceptable to Vietnam," she said, adding that her colleagues had met Chinese embassy officials "to express our opposition to such acts".
On Thursday, hackers from both countries planted patriotic messages on hundreds of websites, including government sites.
It follows anti-China protests by hundreds of Vietnamese over the weekend.
China's ambassador to the Philippines, Liu Jianchao, has insisted China's intentions were peaceful and said that China was not looking for oil in the disputed waters and, therefore, no other country should.
"We're calling on other parties to stop searching for the possibility of exploiting resources in these areas where China has its claims," he told reporters.
"We will never use force unless we are attacked," he said.
The Philippine government has accused two Chinese patrol boats of harassing a Philippine oil exploration ship on 2 March this year.
The Philippines has said it has seen new structures being built on islands which it claims.
"That's part of our exercise of jurisdiction. It's not harassment," Mr Liu said.
He also rejected the involvement of the United States in regional attempts to resolve the long-running territorial dispute.
China prefers to tackle each conflicting claim with each country separately.
Vietnam, Indonesia and the Philippines have led regional efforts to seek a multilateral resolution of the conflict.