When I first landed in Hong Kong for a family holiday a few nights before the clashes with police, we were greeted in the arrivals hall by a large and vocal crowd of protesters, chanting and handing out leaflets.

When we returned to the airport to depart a few days later, there was a marked change. The protesters had swelled from a few hundred to several thousand and there was now a very tense atmosphere. In front of the abandoned check-in desks, a crowd had gathered around a man who was allegedly a police officer posing as a protester. He was, by all accounts, being roughed up and had been cable-tied to a luggage trolley.

Many protesters were appalled by the treatment of this apparent infiltrator. “This will be used against us,” one told me. Apparently, the “policeman” turned out to be just a student, an innocent, but such was the prevailing air of mistrust that he had become the unfortunate victim of a misplaced anger.

[Update 23 August: It later emerged that the man was not a student but a journalist for the Chinese state newspaper Global Times.]

Young students, who made up most of the throng, offered us water and apologised for the inconvenience we were suffering. A member of the local press corps – in hi-vis jacket – advised us to get out of the airport building. “Something’s going to happen,” he said.

Facebook Twitter Pinterest A man who protesters claimed was a Chinese undercover agent is restrained at the airport. Photograph: Vincent Yu/AP

We had just made our way to the top of one of the exit ramps when three busloads of Hong Kong police pulled up. A huge roar went up from the crowd inside, and they began running to the bottom of the ramp we’d just come up. Some 20 blue-shirted officers prepared for the fray with protective goggles, a few batons, and canisters on their hips.

We found ourselves caught between the police and the protesters. The police moved cautiously through us and on to the ramp to confront the protesters. We were presented with an extraordinary sight: a tiny phalanx of officers facing off a huge chanting crowd.

The noise, amplified by the cavernous airport roof, was deafening. The police numbers looked comically small, and the officers looked around nervously. After a few minutes of a stand-off, they began to withdraw, pursued by the crowd. Again, we found ourselves caught between them as the police hurried back into their buses.

Now the anger and frustration boiled over: the buses were surrounded by chanting protesters who began hammering on the windows and hurling plastic bottles. Two buses got away; the third was halted by a rapidly improvised barricade of luggage trolleys. The last bus finally got through, along with an ambulance that was conveying the injured “impostor” student to hospital.

As we were ushered back inside the airport by concerned protesters another huge roar went up. We could now see riot police in full protective gear grabbing and beating protesters from just inside the airport and dragging them outside.

Carried along in a huge sea of people, we were propelled to the fast train service to downtown Hong Kong. At one stage during this protest – now in its 11th week – there were two million people on the streets of Hong Kong. This is nearly a third of the population, which makes this perhaps the largest per capita protest ever, anywhere in the world.

I think there is a real fear here that China is not prepared to wait until 2047, when Hong Kong is due to be amalgamated into the mainland. It wants to speed up that process. Hong Kong citizens can see the way this is going. What began as a protest over an extradition bill has grown into a fight for the future of Hong Kong, its freedom and democratic rights.

One of the leaflets we were given reads: “We’re fighting to put these broken pieces back together, to preserve what makes this city our home... a beautiful Hong Kong... the city we’ll always love.”