Almost guaranteed to be caused by a 3rd party app that is installed to the phone. If you don't know which is causing this I would work backwards from most recently installed app. Uninstall, reboot phone and see if the popups are there. Continue doing this until they are gone, knowing that the last app uninstalled prior to seeing no more popups was the one causing this.
Sounds like you downloaded some malware or an app that is add supported. What are the last few things you installed? Try removing them and see if the problem goes away. Also, some web sites are very skillful at presenting popups that appear to be official updates and such but do nothing other than install software that does exactly what you're describing.