-The brand-new vehicles, complete with real-time surveillance cameras and multiple spray nozzles, were wheeled out after police clashed with demonstrators at nearly a dozen locations on Sunday.
They fired tear gas on shopping streets and in subway stations, with protesters hurling bricks and spraying riot police with fire extinguishers and water hoses.
A government official said 45 people were injured in the clashes, including two who were in serious condition.
Among them was a woman who suffered a serious face injury, reportedly after being hit by a bean bag round, with rumours circulating that she had lost her vision in the incident.
Images of her lying on the ground with blood pouring from her face quickly went viral and featured on posters calling for new demonstrations.
"An eye for an eye" read one call for a protest on Monday afternoon at the city's airport, where thousands of protesters dressed in their movement's signature black gathered holding signs reading "Hong Kong is not safe" and "Shame on police."
Hong Kong carrier Cathay Pacific warned the disruption could cause delays or cancellations of flights.
- Water cannons on show -
Police have defended themselves as accusations of using excessive force against protesters and on Monday unveiled two water cannons -- a method that has not yet been used during the crisis.
They demonstrated jets of water from the trucks on several dummy torsos placed at different distances from the vehicles.
Hong Kong has reportedly ordered three of the vehicles at a cost of HK$27 million ($3.4 million), though police declined to confirm the exact price-tag.
Police would only use the trucks in the event of a "large-scale public disturbance" leading to "casualties, property being destroyed wantonly, or public order and public safety coming under grave threat", senior superintendent Chan Kin-kwok told lawmakers.