A Hong Kong police officer shot at masked protesters on Monday morning -- hitting at least one in the torso -- during clashes broadcast live on Facebook, as the city's rush hour was interrupted by protests.

Footage showed a police officer drawing his sidearm in the district of Sai Wan Ho as he tried to detain a masked man at a road junction that had been blocked by protesters.

Another masked man then approached the officer and was apparently shot in the chest area, quickly falling to the ground, clutching his left side. His condition was not clear.

Seconds later, two more live rounds were fired by the officer during a scuffle and another masked man went to the ground, although the footage was less clear as to whether he was struck.

Police could later be seen detaining the two men on the ground. The first man had a pool of blood next to him, his body limp as officers moved him around and apparently tried to tie his hands.

The second man was conscious and talking.

A police source, speaking on condition of anonymity, confirmed to AFP that live rounds were fired at more than one protester in Sai Wan Ho and that a statement would be issued.

Hong Kong has been upended by five months of huge and increasingly violent rallies, but Beijing has refused to give in to most of the movement's demands.

Tensions have soared in recent days following the death on Friday of a 22-year-old student who succumbed to injuries sustained from a fall in the vicinity of a police clearance operation the weekend before.

The city has seen four days of consecutive protests since the student's death as well as tens of thousands attending mass vigils.

Using online messaging forums, activists had called for a general strike on Monday morning. 

Flashmob protests sprung up in multiple districts during the morning commuter period, with small groups of masked protesters targeting subway stations and building barricades on road junctions.

Even before the shooting in Sai Wan Ho, tear gas had been fired in at least two other locations. 

Monday's shooting is the third time protesters have been shot with live rounds by police. 

On 1 October a student was struck in the chest as he and a group of activists attacked an officer with sticks and poles. He survived his wound and is being prosecuted.

Days later a 14-year-old boy was shot in the leg when a policeman in plainclothes fired his gun after his car was attacked by a crowd. He also survived and was arrested.