He said some of their workers were also arrested on the grounds.
"Yesterday night, we received intelligence of a very heinous plan by Kenyan Kwanza to ferry goons who would attack peaceful attendees of our rally in Kamukunji. These goons had express order to shoot directly to the crowd while enjoying police protection," Raila said.
"Early this morning, our crew who had been sent to set up the stage at Kamukunji were attacked and their equipment damaged. Some were arrested. In line with the intelligence we received, armed goons have this afternoon descended on Kamukunji and shot indiscriminately into the crowd."
ODM Secretary General and Nairobi Senator Edwin Sifuna also blamed the police for interrupting preparations for the Kamukunji rally.
"Absolute madness from Police to disperse a peaceful assembly at Kamukunji. We have held dozens of rallies here and they have always been peaceful. What is the justification for this?," Sifuna said in a tweet.
Slain Ugandan athlete Rebecca Cheptengei accorded military burial