In the capital, Hanoi, demonstrators sang patriotic songs and held up placards opposite the Chinese embassy.
Tensions have been running high after Vietnamese ships clashed with Chinese vessels guarding an oil rig in a contested area of the South China Sea.
The protests appear to have the Vietnamese government's approval.
The country's communist authorities have broken up previous anti-China demonstrations because of fears that they may be hijacked by pro-democracy activists, says the BBC's Asia Pacific editor, Charles Scanlon.
Nevertheless, he says, Hanoi has also used the demonstrations to communicate its anger over what it sees as Beijing's aggressive infringement of Vietnamese sovereignty.
The protesters opposite the Chinese embassy on Sunday included war veterans and students.
"This is the largest anti-Chinese demonstration I have ever seen in Hanoi," a war veteran named Dang Quang Thang told the AFP news agency.
"Our patience has limits. We are here to express the will of the Vietnamese people to defend our territory at all costs. We are ready to die to protect our nation," he is quoted as saying.
Large anti-China protests were also seen in other Vietnamese cities.
Earlier this month, ships from the two countries collided near a Chinese oil drilling platform in the South China Sea.
China has warned Vietnam to withdraw its ships from waters, off the disputed Paracel Islands, that it claims as its own.
But Vietnam - which also claims that stretch of sea - accused China of having sent 80 vessels, including navy ships, to support an oil drilling operation.
It released video footage to back its claim that Chinese ships had rammed Vietnamese vessels.
The US has accused China of provocation, and warned that the dispute could destabilise the region.
The issue was also discussed by foreign ministers at the 10-member ASEAN summit of south-east Asian Nations in Myanmar (also known as Burma).
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Hundreds of people across Vietnam have protested against China's role in a sea dispute - the largest rallies of their kind recently in the communist country.