HONG KONG — China rotated troops in its People's Liberation Army garrison in Hong Kong on Thursday, days before protesters planned to hold a march in the Chinese-ruled city.

Chinese state media described the troop movement as routine but the timing is likely to hit nerves in the former British colony, which was returned to China in 1997, that has seen three months of sometimes violent demonstrations.

China's military will make even greater contributions to maintaining Hong Kong's prosperity and stability, state news agency Xinhua cited the People's Liberation Army (PLA) garrison in the territory as saying.

The military had completed a routine troop rotation of air, land and maritime forces, the news agency said. Xinhua and the People's Daily released pictures and footage of armored personnel carriers moving in convoy in Hong Kong before dawn, their lights flashing.

Earlier this month, hundreds of People’s Armed Police conducted exercises at a sports stadium in Shenzhen that borders Hong Kong a day after the U.S. State Department said it was “deeply concerned” about their movements.

People's Liberation Army (PLA) troops from the Hong Kong Garrison arriving on during a routine rotation of personnel on Wednesday. CCTV / AFP - Getty Images

Observers estimate the Hong Kong garrison numbers between 8,000 and 10,000 troops split between bases in southern China and former British army barracks in Hong Kong. Past statements from China said the number of soldiers in Hong Kong "was maintained with no change."

That was not in Thursday's announcement.

Chinese defense ministry spokesman Ren Guoqiang told a regular monthly news briefing that the timing of the troop rotation was similar to that of previous years to "meet the demands of defending Hong Kong."

The garrison troops would fulfill their obligation of defending Hong Kong according to the law and would follow the orders of the Communist Party, he added.

They had the confidence, determination and capability to "protect and defend Hong Kong's long-term prosperity and stability."

He did not answer a question on whether troop levels in Hong Kong had risen as a result of the new troops arriving.

The Civil Human Rights Front, the organizer of previous mass protests in Hong Kong that they said attracted up to 2 million people, plans a rally from Hong Kong's Central business district to Beijing's main representative Liaison Office in the city on Saturday.

Unrest escalated in mid-June over a now-suspended extradition bill that would have allowed people to be sent to mainland China for trial in Communist Party-controlled courts.

It has since evolved into calls for greater democracy under the "one country, two systems" formula under which Hong Kong has been run since 1997, guaranteeing freedoms not enjoyed on the mainland that include an independent judiciary.

The protests have posed the biggest challenge for Communist Party rulers in Beijing since President Xi Jinping took power in 2012.

Beijing is eager to quell the unrest before the 70th anniversary of the founding of the People's Republic of China on Oct. 1, when Xi will oversee a large military parade in the Chinese capital.