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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Marvyn Iheanacho, 39, attacked Alex Malcolm in Mountsfield Park, Catford, on 20 November 2016.
Witnesses heard a "child's fearful voice saying 'sorry'", loud banging and a man screaming about the loss of a shoe, Woolwich Crown Court was told.
Iheanacho, who denied murder, will serve a minimum of 18 years in prison.
The jury heard Alex suffered head and stomach injuries and died in hospital two days after the beating.
His mother, Lilya Breha, 30, told the court Iheanacho had also attacked her after she tried to call an ambulance when he returned to her flat carrying her injured son.
In a tearful interview, she said: "He (Alex) was bubbly. He was just perfect you know, he was a really, really special little boy.
"He was shy, he was so shy, and very polite. He would always say 'Mummy thank you' and 'I love you Mummy'."
Ms Breha described Iheanacho as a "good liar" and pathetic.
Recalling how she met him through a friend after he left prison, she said he had convinced her he was innocent and a good person.
She said: "When I think about it now, to be honest I feel like it was all such a big lie and he just pretended to be a good guy pretty well."
Prosecutors revealed there had been "problems with witness interference" during the trial, with Iheanacho phoning Ms Breha from prison to try to persuade her to back him in court.
In a victim impact statement, Alex's father said he would "never forget seeing him in his hospital bed fighting for his life. That image will stay with me forever."
"Just thinking about what's happened and trying to put words on paper is tearing me apart," he said.
Sentencing, Judge Mark Dennis QC said the killer had a deeply entrenched character flaw that "leads you to overreact and lose your temper".
"You used your undoubted strength and simple brute force," against a "completely defenceless" child, he said.
He said Iheanacho, who has a string of previous convictions for violent offences, had given fake and misleading accounts to paramedics, hospital staff and police.
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.
[NEXT_CONCEPT]
A man who battered his girlfriend's five-year-old son to death in a London park for losing a trainer has been jailed for life.