Potty Training

A week ago today we started potty training Zach. We had tried many times in the past 9 months but each time is was obvious that he wasn't ready. This time though I was determined. He is going to be 3 in just a couple months and I felt like it was past time to get rid of the diapers.
So last Wednesday, Keith's birthday, we pulled out the underwear and the potty and got started. We started out putting him on the potty every 15 minutes but several times as soon as he got off he would pee in his underwear. About 2 in the afternoon we caught him starting to go and rushed him onto the potty. After that things got better. He stayed dry the rest of the evening and only had a couple accidents the next day. But in the last few days we have gone backwards. He has had several accidents and if you ask him if he needs to use the potty he screams NO even when he needs to go. If we sit him on the potty he throws a fit and cries the whole time.
It's so frustrating since I know that he knows when he is going and he just keeps fighting it.