I'm having the same issue, though I'm upgrading from ios7.
When I plug my iPhone into my Mac Pro to try and reverse the issue with iTunes, iTunes provides the following pop-up message:
"To allow access, please respond on your iPhone..."
The message suggesting I should simply follow the prompt on my iPhone to 'slide to upgrade'. Of course, as per the OP's issue, my screen is unresponsive.
I can turn my iPhone off. When it comes on again I see the following screens:
[1] 'slide to upgrade': at this point the screen is responsive. When I slide my finger over the screen as per requested I'm greeted with my password screen. I am able to enter my password.
Then,
[2] The Apple logo appears with a loading bar underneath. The loading bar disappears and then I see the following:
[3] 'Hello' > 'slide to set up'
Again, the screen is responsive and I'm taken to another screen:
[4] Update Completed: your iPhone was updated successfully. There are just a few more steps to follow, and then you're done! <Continue>
I hit <Continue> and am taken to the 'slide to upgrade' screen, from which point the screen is no longer responsive.
Please advise.