How to set web.config file to show full error message

I deployed my MVC-3 application on windows Azure. But now when I am requesting it through staging url it shows me (Sorry, an error occurred while processing your request.). Now I want to see the full error message, by default it is hiding that because of some security reasons. I know that we can do this through web.config file. But how?

Not sure if it'll work in your scenario, but try adding the following to your web.config under <system.web>:

<system.web>
    <customErrors mode="Off" />
    ...
</system.web>

works in my instance.

also see:

CustomErrors mode="Off"

This can also help you by showing full details of the error on a client's browser.

<system.web>
    <customErrors mode="Off"/>
</system.web>
<system.webServer>
    <httpErrors errorMode="Detailed" />
</system.webServer>