How to fix R cannot be resolved to a variable error in Eclipce, Android Studio?

Question

Android

I created a new package and moved all my activities there by simple drag and drop. in the manifest made the new package the main one. After that, the error R cannot be resolved to a variable appears. Tried clean, build project. Checked xml files for errors - nothing in the end. This creates a new R.java file. Please tell me how to fix it.
PS. Created a new project and it immediately got the same error.