The police toss smoke grenades, not tear gas grenades, a short distance upwind, between themselves and the protestors.
This is not riot control, it is a manufactured photo opportunity
Observe the hand motion. He is tossing a smoke grenade just in front, not at the protestors. If you are wondering how heavily outnumbered protestors accomplished their goal of occupying the city hall, despite announcing it at least a day in advance, the above photo explains the inexplicable.