EThis is the last time in Hong Kong.
Chinese intervention would essentially confirm that the ruling Communist Party has failed to win the hearts and minds of the seven million people in the form of the British colony, and risk severely damaging the city's image as a global financial hub.
Protesters first took over the streets of China, where Communist Party control has a 99.9 per cent conviction rate.
Their calls to have grown up to be bigger political reforms, including direct leadership elections, and anger is rising against the backdrop of massive crowds.
On Thursday, the Chinese People's Armed Police conducted a sports stadium in Shenzhen, as the US State Department expressed concern that they could be deployed across the border in Hong Kong to break up protests wracking the city.
Men in fatigues could be seen at the Shenzhen Bay Sports Center, and shouts and whistles could be heard at Reuters journalist on Thursday morning.
Later in the day, the police in one of the groups, one wearing black t-shirts in Hong Kong.
The other group remained in uniform and picked up crowd-control shields and practice charging at the first group.