I had the same problem, quite weird because it was happening only when using Eclipse (but it was OK with Ant). This is how I fixed it:
Right click on the Project Name
Select Build Path -> Configure Build Path
In Java Build Path, go to the tab Order and Export
Uncheck your .jar library
Only sometimes: In Order and Export tab I did not have any jar library there, so I have unchecked Android Private Libraries item. Now my project is running.