HANOI, Vietnam >> Vietnam smothered anti-China protests on Sunday with a massive security clampdown after deadly riots triggered by a territorial dispute with Beijing spooked foreign investors and the country’s authoritarian leadership alike.

As patrol ships from both countries remained locked in a standoff close to a Chinese oil rig in a disputed patch of the South China Sea, Beijing said it had evacuated 3,000 nationals from Vietnam and was sending the first of five ships to pull out others wanting to leave.

China also said that it would suspend some of its bilateral exchange plans with Vietnam and that it was advising Chinese not to visit the country.

China’s decision to deploy the massive oil rig on May 1 has been widely seen as it one of its most provocative steps in a campaign to assert its sovereignty in the waters. It triggered fury in Vietnam and the worst breakdown in ties between Hanoi and Beijing in years.

Tensions have been mounting between the two countries despite their sharing of a political ideology. Both nations are run by communist regimes that since the 1990s have embraced free market capitalism while retaining large state sectors and powerful internal security systems.

Last weekend, Vietnam permitted anti-China protests that drew thousands of people, a rare step that allowed it to amplify state anger against Beijing. Doing so was risky for authorities: Dissident groups joined the protests, and by Tuesday and Wednesday, the rallies had morphed into riots targeting factories believed to be owned by Chinese companies, though many of those hit were Taiwanese. Two Chinese nationals were killed and more than 100 wounded.

Vietnam’s state-security apparatus on Sunday ensured no one was able to protest, with thousands of police and security officers flooding southern Ho Chi Minh City and the capital, Hanoi. Police were posted outside well-known dissidents’ houses, preventing them from leaving, according to activists.

In Ho Chi Minh City, police detained several demonstrators after dragging them from a park close to the city’s cathedral. Authorities in Hanoi closed off streets and a park close to the Chinese Embassy, while police barking into bullhorns shoved journalists and protesters away.

“I want to send a message that if we don’t stop China today, tomorrow it will be too late,” said demonstrator Dao Minh Chu, as he was pushed away from the park near China’s embassy, where last week around 500 people gathered without interference from authorities. Those protests were covered enthusiastically by state media, a clear sign of state sanction.

Some users in Vietnam on Sunday reported having trouble accessing Facebook, a popular medium for Vietnamese to get news and photos of demonstrations from activists. The government keeps a low-level and sporadic block on popular social media platforms.

China has loudly demanded that Hanoi protect Chinese people inside Vietnam, which is heavily dependent on Beijing economically. Hundreds of Chinese have left by commercial flights and across the land border into Cambodia, although there has been calm since Thursday.

On Sunday, China said it dispatched to Vietnam a passenger ship capable of carrying 1,000 people, the first of five vessels it planned to send to complete an evacuation on top of 3,000 nationals who had left earlier. With Chinese traveling in increasing numbers, Beijing is under pressure to protect them overseas.

China’s Foreign Ministry said two charter flights carrying more than 290 Chinese employees from affected businesses arrived in Chengdu in southwest China on Sunday afternoon. They included more than 100 injured people.

The ministry also said that the government would suspend some of its bilateral exchange plans with Vietnam and that it was advising Chinese not to visit the country.

In a statement posted on the ministry’s website, spokesman Hong Lei said the violence that has resulted in Chinese casualties and property losses had “damaged the atmosphere and conditions for exchanges and cooperation between China and Vietnam.”

For the time being, China is advising its citizens not to travel to Vietnam and has suspended some bilateral exchange plans, and will take further measures if necessary, Hong said.

No details were given on the bilateral exchange plans.

A Taiwanese steel mill attacked on Wednesday employed 1,000 Chinese workers, who can be cheaper to hire and easier to manage than Vietnamese laborers.

Yang Yang, a political scientist at the China University of Political Science and Law in Beijing, said there were so many Chinese working in Vietnam that sending ships might be more practical than planes. “It can also appease the unhappiness of the Chinese public over the violence against Chinese nationals in Vietnam,” he said.

In recent years, foreign companies attracted by low wages and a reputation for safety have flocked to Vietnam, opening factories making everything from sneakers to smartphones. The government is aware that last’s week violence threatens that vital economic cog.

On Saturday, top Vietnamese security official Lt. Gen. Hoang Kong Tu vowed to ensure the safety of all foreign investments and citizens in the country, including those from China. More than 1,000 people have been arrested in connection with the violence, which authorities have blamed on “extremists.”

While China and Vietnam have growing business links and share a political ideology and a commitment to authoritarianism, they also have a long history of bad blood. Many Vietnamese harbor deep resentment over what they see as China’s bullying and economic exploitation of Beijing’s far smaller neighbor.

China has been much more assertive in pressing its territorial claims in recent years, but the placement of the rig 220 kilometers (136 miles) off the coast of Vietnam was considered especially provocative.

Hanoi sent patrol ships to confront the rig and scores of Chinese vessels protecting it, and they remained locked in a tense standoff. Neither side has shown any sign of withdrawing their ships or willing to compromise.

Vietnam’s government routinely arrests free speech activists and others challenging one-party rule, and anti-China protests have been one of the few opportunities for public gatherings. But several well-known activists said they had been prevented from leaving their homes on Sunday.

“I think the best way is to allow people to protest,” said La Viet Dung, a frequent anti-China protester, adding that police visited him late Saturday asking him not to attend. “They say they are preventing people from protesting because they are worried about extremist actions and violence, but that is not logical.”

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Dinh reported from Ho Chi Minh City, Vietnam. Associated Press writer Louise Watt and news assistant Henry Hou in Beijing contributed to this report.