"liberate" a fire truck (water pumper); drive it to protest site.  Sprinkle tear-gassed areas liberally with water, clearing and cleaning the air.  Note: some water may also happen to reach public "security forces".  This is normal collateral advantage.

   Add some: cooking oil, olive oil or other non-toxic oils to the street surfaces.  It impedes others' being able to stand and hold positions easily; they might also lose their grips on various things such as weaponry, shields, truncheons, etc.  What a shame that would be.

  (colorful) Confetti is colorful (uh, right?  ;^)  ), presents a festive air and creates a mental dissonance with the "security forces' " physical violence.  

 the common warehouse (and truck delivery) type hand-cranked pallet-lifts can (with the help of a handy crowd of assistants) also move cars into barricade formations and thus help isolate unhelpful "security forces".  

   Protesters wearing hospital, surgeon, etc., garb makes for another physically dissonant presentation when "security forces" find themselves faced with using physical violence against people dressed in such apparel.

   conjure and add your own ideas; circulate, there's a world of possibilities.

 NOTE:  Use/adapt all suggestions to circumstances and with prudent fore-thought for responses and reactions by crowds and police forces.

"In such an environment it is not surprising that the ills of technology should seem curable only through the application of more technology..." John W Aldridge