The video clip below shows, from about 1min 40secs, the Pitak Siam protesters beginning to attack the police. At 1min 50secs a white gas can clearly be seen beginning to blow amongst the Pitak Siam supporters and then at 1min 53secs an object, trailing the same white gas, is thrown at the police, causing them to disperse slightly. Keep watching until for another 30seconds or so and it is even more clear the original gas grenade thrown by Pitak Siam is spreading out amongst the police. Furthermore a simple count of gas trails between the two camps shows, initially, a higher number of gas grenades being thrown from the protesters side towards the police. At the very least it appears the protesters had their own gas canisters.
It would be pretty hard for any credible and verifiable source to debunk this new evidence and it now seems almost certain that Pitak Siam were responsible for escalating the confrontation.
Bangkok Pundit also mentions the tear gas attack in his recent post on the rallies and concludes
It is hard to tell exactly who started firing/throwing the tear gasMore bizarrely regarding the tear gas attack Bangkok Pundit cites an anonymous internet troll and cyberstalker who usually specialises in the internet stalking of women using threatening graphic sexual commentary and extreme racism, samples of which can be found here and here. Maybe Bangkok Pundit will cite Violentacrez in his next post on Thailand's rice pledging policy?
On balance readers have to make their own judgement. It seems self-evident that Pitak Siam were intent on hurting and attacking police officers and given the other evidence - such as protesters driving a truck directly into the police officers - the likelihood of them throwing teargas (weapons of all kind are easily accessible in Thailand) at the police is apparent.