Arsenal took the lead in controversial fashion midway through the first half as Spain midfielder Cazorla flung himself to the floor following a challenge by Steven Reid, with replays showing there was no contact. Mikel Arteta converted the resulting penalty.
"There was zero contact, it wasn't even close. I am not sure what the referee saw," said the angry West Brom manager.
"It was a bad decision for us and obviously changed the shape of the afternoon.
"It could have been a nervous afternoon for Arsenal, at 0-0 they hadn't created too much.
"We knew the longer the game went the more pressure would come on Arsenal and then we would have a chance. That decision changed it."
Clarke continued: "The second was a clear penalty, no complaints, Chris fouled Oxlade-Chamberlain - but Goran Popov was fouled in the lead up to it and for some reason the referee, who gave soft free-kicks all afternoon, decided not to give that one.
"Another poor decision for us, you can add the [Per] Mertesacker handball in the first half - it was a normal ball into the box and somehow the officials decide it was a free-kick as it's the easy way out.
"They gave it for a free-kick on somebody but I've seen the replay and for the life of me cannot see a free-kick."
Clarke stopped short of calling Cazorla a cheat, though, adding: "There is a share of culpability, but on that one it is so clear there was no contact, the referee has to be better with his decision.
"I don't want a witch hunt on referees, in his country we have fantastic referees and a fantastic group that work hard to make the game the game it is.
"We have to try to work as hard as we can with the referees, in this game unfortunately it was the poorest of a really good group.
"We have a number of good referees, we have to find good more referees. That is the way forward."