Who said 'voice of the people is voice of god'?

Julius Caesar, but I think it might have been a quote from someone earlier. The "vox populi, vox dei" has been requoted in many times and places.

---- A Supplementary Observation

The earliest reference I know of to vox populi vox dei is Alcuin's letter to Charlemagne.

Alcuin quotes it as proverbial, so someone must have said it before him. But as far as I am aware nobody knows who Alcuin got it from.