SHENZHEN, China/HONG KONG (Reuters) - Hundreds of China’s People’s Armed Police conducted exercises at a sports stadium in Shenzhen on Thursday, as the U.S. State Department expressed concern that they could be deployed across the border in Hong Kong to break up protests wracking the city.

But Western and Asian diplomats in Hong Kong said Beijing has little appetite for rolling the PAP or the People’s Liberation Army onto Hong Kong’s streets.

Men in fatigues could be seen in a stadium at the Shenzhen Bay Sports Centre, and shouts and whistles could be heard by a Reuters journalist on Thursday morning.

Later in the day, police carried out exercises in which they divided into two groups, one wearing black t-shirts similar to those worn by some protesters in Hong Kong.

The other group remained in uniform and picked up crowd-control shields and practised charging at the first group.

“This is the first time I’ve seen such a large-scale meeting,” said Yang Ying, a receptionist at a wellness center inside the retail complex at the stadium.

“There have been exercises in the past, but usually they involve traffic police,” she added. “Our friends, social media, all say it’s because of Hong Kong.”

Ten weeks of increasingly violent confrontations between police and protesters have plunged the city into its worst crisis since it reverted from British to Chinese rule in 1997.

The protests represent one of the biggest challenges for Chinese President Xi Jinping since he came to power in 2012.

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On Wednesday the U.S. State Department said it was deeply concerned about reports that Chinese police forces were gathering near the border with Hong Kong and urged the city’s government to respect freedom of speech.

In Shenzhen, paramilitary police marched in and out of the stadium near a retail complex where shoppers milled about.

The stadium parking lot was filled with more than 100 dark-painted paramilitary vehicles, including troop trucks, armored personnel carriers, buses and jeeps. At least three were armored front-end loaders, and two vehicles carried water cannons.

A policeman patrolling the area told Reuters journalists the paramilitary police were guarding a basketball tournament.

Chinese state media have made several mentions of exercises in Shenzhen.

The Global Times, a nationalistic tabloid run by the ruling Communist Party’s official People’s Daily, published a slick video this week showing columns of trucks and armored personnel carriers rolling through the city.

The paper said the vehicles belonged to the People’s Armed Police and had gathered for “apparent-large scale exercises.” Its editor, Hu Xijin, described it on Twitter as “a clear warning to rioters in Hong Kong.”

The Communist Party’s official People’s Daily newspaper said on the social media platform Weibo that the force handles incidents that include riots and terrorist attacks.

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“There is usually PAP in Shenzhen, but this time it appears to be directed against Hong Kong,” said a taxi driver, who gave his surname as Wang. “Usually they wouldn’t hold such a big exercise.”

POSTURING, FOR NOW

Still, diplomats in Hong Kong said they believe China’s leadership is well aware that moving mainland forces into Hong Kong would shatter international faith in the “one country, two systems” model at the heart of the city’s role as a global financial hub, and would swiftly lead to sanctions.

“We are seeing an escalation in the posturing that appears designed to send messages to both Hong Kong and the mainland ... but we are confident we are still in the realms of propaganda here,” one senior Western envoy told Reuters, speaking on condition of anonymity because he was not authorized to speak to the media. “The trucks aren’t about to roll at this point.”

Alexander Neill, a Singapore-based security analyst at the International Institute of Strategic Studies, said it would take a far more serious situation to trigger such a move.

“I think we’d really need to see a total breakdown and a sense that China’s national interests were at great risk and could not be protected by the Hong Kong government and its police,” he said.

Worst-case scenarios, he said, could involve the assassination or kidnapping of Chinese officials in Hong Kong, the sacking and occupation of key offices there or the government losing control of parts of the police force.

“We’re a long way from anything like that yet, so I think what we are seeing is harder posturing that is part of a much broader propaganda effort underway on many fronts,” he said.

The Shenzhen stadium sits across the water from Hong Kong’s rural hinterland, near a bridge straddling the border.

People working around the stadium said the level of PAP activity was higher than they had ever seen, but not disruptive, as the troops usually kept to themselves.

The chief disruption was PAP vehicles occupying parking spaces, and people who worked in the area said they had been told not to share pictures of vehicles.