Hong Kong -- Things were much calmer at Hong Kong International Airport on Wednesday, as tight new restrictions came into effect after a night of violent clashes between pro-democracy protesters and police. But the calm belied mounting anger on both sides, as China called the chaos of the previous evening an atrocity, and the protest an act of terrorism.

CBS News correspondent Debora Patta reported that newly released satellite photos appear to show Chinese troops massing at the border. The August 12 images show about 100 vehicles -- apparently armored personnel carriers and other military vehicles -- lined up in rows inside a sports stadium in the city of Shenzhen, which sits just inside mainland China on the border with semi-autonomous Hong Kong.

China has said the military build-up is just to conduct drills, but Hongkongers and many abroad see it as a direct warning to the pro-democracy movement that Beijing is ready to send troops in to quell the months-long demonstrations if necessary.

An Aug. 12, 2019 satellite image provided by Maxar Technologies shows what appear to be Chinese military vehicles lined up inside a sports stadium in the city of Shenzhen, mainland China, on the border with Hong Kong. Satellite image ©2019 Maxar Technologies

U.S. reacts to crackdown

President Trump noted the Chinese military build-up in a tweet on Tuesday. "Everyone should be calm and safe!" he wrote.

A spokesperson at the State Department went further in a statement later in the day, urging China to "adhere to its commitments" made when it took control of the city back from Britain in the 1990s.

The statement noted that Beijing had granted Hong Kong "a high degree of autonomy," and called on the local administration in the city, which is appointed and supported by Beijing, "to respect the freedoms of speech and assembly, as enshrined" in the handover agreement.

Our Intelligence has informed us that the Chinese Government is moving troops to the Border with Hong Kong. Everyone should be calm and safe! — Donald J. Trump (@realDonaldTrump) August 13, 2019

"We condemn violence and urge all sides to exercise restraint, but remain staunch in our support for freedom of expression and freedom of peaceful assembly in Hong Kong," the State Department spokesperson said, adding: "these freedoms must be vigorously protected."

Accused spies attacked

The ongoing demonstrations reflect the broad concerns in Hong Kong that the city's cherished autonomy is being eroded by Beijing.

Armed with pepper spray and swinging batons, riot police stormed Hong Kong's International Airport on Tuesday night, initially trying to help first responders reach two injured men.

Defiant protesters gave chase. They seized an officer's baton and turned it on him, only retreating when he stumbled back and pulled out a gun.

An injured man who was suspected by protesters of being a Chinese spy is taken away by paramedics at Hong Kong International Airport, early on August 14, 2019. Getty

Protesters were convinced the two injured men were undercover officers from mainland China, attempting to pass off as members of the pro-democracy movement. Accused of being spies, the two injured men felt the full brunt of the protesters' rage. In two separate incidents they were kicked, beaten, punched and drenched in water. One of the man was bound with cable ties and left on the ground in a fetal position.

Finally emergency workers were allowed take them away.

On Tuesday, Hong Kong's Airport Authority announced new security measures barring anyone without a ticket or valid employee identification from entering the airport buildings. There were still a few hundred protesters inside the arrivals hall, who gained access before the new restrictions were put in place. They were speaking to travelers, but remained calm.

An apology. Too late?

Hong Kong lawmaker Fernando Cheung rushed to the airport to assist, arriving as the violence erupted.

"We are in a situation where people are getting ready to fight and many young people are ready to sacrifice themselves, and we do not want to see that happen," he said.

For two consecutive days protesters paralysed the airport's operations. Hundreds of flights were cancelled as the black-clad demonstrators choked both arrivals and departure halls. Some travellers were furious, shouting at the protesters and accusing them of being unfair and selfish to people who were just trying to work or see loved ones.

In the cold light of day, the protest leaders realized their actions might have backfired. On Wednesday they handed out apology letters at the airport.

"Please accept our sincere apology to all press travelers, press reporters, paramedics," the letters said. "We will learn from our mistakes."

A letter of apology issue by pro-democracy protesters in Hong Kong on Aug. 13, 2019, after violent clashes with police at Hong Kong International Airport snarled transport and left at least two people injured the previous night.

"We apologize for our behavior but we are just too scared," read another letter. "Our police shot us, government betrayed us, social institutions failed us. Please help us."

But a collision course has now been set. Police have been using force far more frequently as a first resort in recent days, and it doesn't take much to trigger protesters' hatred for them.

China can only intervene in Hong Kong's affairs if the city's government requests troop assistance. Most Hongkongers still believe that would be a step too far for China, and a Tiananmen Square-style crackdown is unlikely.