It came as the Asian financial center continues to grapple with weeks of protests, that sometimes turned violent. Hundreds of thousands have poured into the streets of Hong Kong since early June to rally against a now-suspended bill.
Earlier, state media reported that troops from the People's Liberation Army (PLA) had moved into Hong Kong. The military's moves were described as routine.
On Thursday morning, the PLA's overseas public relations arm issued a statement that said: "The Hong Kong Garrison will resolutely follow the instructions of the Central Government and the Central Military Commission, resolutely implement the 'one country, two systems' principle."
Hong Kong, a former British colony, returned to Chinese rule under a "one country, two systems" principle in 1997. Under that formula, its citizens enjoy some legal and economic freedoms that mainland Chinese don't have.
The garrison will "resolutely safeguard national sovereignty, security, and develop interests, effectively perform duties to defend Hong Kong, and make important new contributions to safeguarding Hong Kong's prosperity and stability," the statement said, according to a CNBC translation.
Earlier, state-run Xinhua news agency reported that the Hong Kong garrison had "conducted the 22nd rotation of its members in the wee hours of Thursday," Xinhua said.
It published photos of armed personnel carriers and trucks carrying troops at the border, as well as a small naval vessel arriving in Hong Kong.
The troops have been based in Hong Kong since 1997.
But as the months-long protests in the city got increasingly violent, troops and tanks started assembling earlier this month in Shenzhen, a Chinese city bordering Hong Kong, and the military's movements on the ground were closely watched.
China's troop movements on Thursday are "the latest signal that Beijing remains very worried about ongoing Hong Kong protests and the failure of Hong Kong's leadership so far to address what remains a growing political and economic crisis," said Milken Institute Asia fellow Curtis S. Chin.
"The continuing unintended consequence of Beijing intervention in Hong Kong affairs remains the slow death of what has been one of Asia's premiere financial hubs," Chin, who is also the former U.S. ambassador to the Asian Development Bank, told CNBC in an email.
The rallies in Hong Kong were initially spurred by opposition to a proposed bill that would allow people in Hong Kong to be extradited to mainland China, but it has since evolved into a broader movement calling for democracy.
On Sunday, Hong Kong police reportedly drew their guns after protesters attacked officers with sticks and rods, and brought out water cannon trucks for the first time — an escalation in the showdown with protesters.
— Reuters and Associated Press contributed to this article.