Three goals came in a five-minute spell before Rauwshan McKenzie scored in his own net.
Marcelo Sarvas scored for LA in the 65th minute. A poor headed clearance by McKenzie popped the ball out to Landon Donovan, who played a short pass to Sarvas and the midfielder struck past Donovan Ricketts.
Portland equalized almost immediately. Jorge Villafana crossed to Fenando Adi and although the Nigerian's header was blocked by Robbie Rogers, the ball fell perfectly for Diego Valeri, whose powerful rising shot took a slight deflection as it found the inside of the far post.
A second soon followed the first. Another Villafana cross fell to Valeri, but the Argentine's effort was blocked. It then fell to Villafana, following up the play, to tuck his shot into the low near corner.
McKenzie will want to forget his 86th-minute own goal. A Donovan chip allowed Baggio Husidic to cross through the goalmouth and McKenzie's touch was the one he would have least preferred as he knocked the ball by Ricketts.