Mainlanders were urged yesterday to put on hold plans to visit Vietnam as Hanoi clamped down on anti-China protests.
The foreign ministry in Beijing also announced it would suspend some bilateral exchanges after violence last week that left at least two Chinese dead and more than 100 injured.
"China will consider taking further measures according to how the situation develops," foreign ministry spokesman Hong Lei said.
Watch: Over a hundred join anti-China protest in Ho Chi Minh City
In Hanoi and Ho Chi Minh City, large numbers of police with batons cordoned off demonstrators angered by China's establishment of an oil rig in disputed waters near the Paracel Islands in the South China Sea.
In Hanoi, authorities closed off streets and a park close to the Chinese embassy. Police were posted outside dissidents' houses, preventing them from leaving, said activists.
"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.
He was forced away from the park where last week about 500 people gathered without any interference from the authorities.
Those protests were covered enthusiastically by state media - a sign that they had been tacitly approved by the government.
But yesterday, the authorities maintained a tight grip.
In Ho Chi Minh City, a group of mostly middle-aged men clapped and shouted "Vietnam" as they attempted to march to the fortified Chinese consulate.
Police stopped them after minor clashes broke out. One witness said some protesters had tried to display banners critical of the Vietnamese government and were detained.
Police and plainclothes officers prevented reporters from talking to protesters. Even after the initial demonstration had been dispersed, a police presence was maintained throughout Ho Chi Minh City and vans with loudspeakers called for calm.
A political columnist in Ho Chi Minh City said the demonstrators had miscalculated the government's reaction.
"They could not demonstrate as they had planned," he said.
Protests on Tuesday were followed by days of violence across 22 of Vietnam's 63 provinces, with Chinese factories and nationals targeted. Factories owned by other foreign companies were also attacked.
But the authorities seemed eager to prevent a repeat of the riots yesterday, amid rumours of protests on social media.
Police units were deployed at the gates of industrial parks around Ho Chi Minh City on Saturday night, said a Taiwanese businessman who had been coordinating security efforts with other managers. "The situation is very quiet now," he added.
But despite the heightened security, Beijing started mass evacuations on Saturday and these continued yesterday.
Several Chinese airlines and travel agencies announced late on Saturday that charter flights and tours to Vietnam would be suspended and refunds paid.