American diplomatic staff have left their consulate in the Chinese city of Chengdu, after a 72-hour deadline expired.
China ordered the closure in response to the US closing the Chinese consulate in Houston, Texas, last week.
Before Monday's deadline, staff were seen leaving the building, a plaque was removed, and a US flag was lowered.
China's foreign ministry said Chinese staff entered the building after the deadline and "took over".
A US State Department spokesperson said: "The consulate has stood at the centre of our relations with the people in Western China, including Tibet, for 35 years.
"We are disappointed by the Chinese Communist Party's decision and will strive to continue our outreach to the people in this important region through our other posts in China."
As the US consulate closed, crowds of local residents gathered outside, with many waving Chinese flags and taking selfies.
Beijing ordered the closure in response to the US closure of the Chinese consulate in Houston - over claims that it was used as a hub for spying.
Tensions have been escalating between the two countries over a number of issues:
- US President Donald Trump's administration has clashed repeatedly with Beijing over trade and the coronavirus pandemic
- Washington has also condemned the imposition by China of a controversial new security law in Hong Kong
- Last week, a Singaporean man pleaded guilty in a US court to working as an agent of China
- Also last week, four Chinese nationals were charged in a separate case with US visa fraud for allegedly lying about serving in China's military
Chinese state media showed pictures of lorries leaving the US consulate, and workers removing diplomatic insignia from the building.
On Monday morning, state broadcaster CCTV posted a video online of the US flag being taken down.
Dozens of Chinese police were deployed outside the building, urging onlookers to move on.
However, boos were heard when a bus with tinted windows left the building on Sunday, the AFP news agency reports.
When Chinese diplomats left their mission in Houston last week they were jeered by protesters.
The Chengdu consulate - established in 1985 - represented US interests over a vast area of south-western China, including the autonomous region of Tibet, where there has been long-running pressure for independence.
The majority of the diplomatic mission's more than 200 employees were hired locally.
With its industry and growing services sector, Chengdu is seen by the US as providing opportunities for exports of agricultural products, cars and machinery.
After the mission's closure the US will have four consulates in mainland China and an embassy in the capital Beijing. It also has a consulate in Hong Kong, the former British colony.
What happened in Houston last week?
China lost its Houston mission last week, but still has four other consulates in the US and an embassy in the capital Washington DC.
After a 72-hour deadline for Chinese diplomats to leave the Houston consulate expired on Friday, reporters saw men who appeared to be US officials force open a door to enter the premises.
US Secretary of State Mike Pompeo explained the Houston closure, saying Washington had acted because Beijing was "stealing" intellectual property.
Chinese Foreign Ministry spokesman Wang Wenbin responded that the US move was based on "a hodgepodge of anti-Chinese lies".
Why is there tension between China and the US?
There are a number of things at play. US officials have blamed China for the global spread of Covid-19. More specifically, President Trump has alleged, without evidence, that the virus originated from a Chinese laboratory in Wuhan.
And, in unsubstantiated remarks, a Chinese foreign ministry spokesman said in March that the US military might have brought the virus to Wuhan.
The US and China have also been locked in a tariff war since 2018.
Mr Trump has long accused China of unfair trading practices and intellectual property theft, but in Beijing there is a perception that the US is trying to curb its rise as a global economic power.
The US has also imposed sanctions on Chinese politicians who it says are responsible for human rights violations against Muslim minorities in Xinjiang. China is accused of mass detentions, religious persecution and forced sterilisation of Uighurs and others.
Beijing denies the allegations and has accused the US of "gross interference" in its domestic affairs.
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