Why do I have voltage on a plumbing ground strap, and is it dangerous?
You're lucky you haven't been hurt!  

It sounds from your description like the ground wire came off the pipe clamp 



If I was you I would definitely not touch that again.  Under normal circumstances that wire should not carry any current.  If you are seeing sparks, the ground wire is energized and carrying current.  

If it does, it may indicate an extremely dangerous condition.  It can shock you (possibly seriously injuring or killing you), start a fire, and damage electrical devices in the house.  The shock hazard isn't just with the loose wire, it may be there with incidental contact with the pipes, sinks, etc. in the house.  It's not really safe to be in the house.  

If it is what it sounds like to me, you pulled the ground wire loose from its lug on the ground clamp - it shouldn't come loose easily, so maybe the wire was damaged, or the screw was not tight.  In any case you didn't cause this problem by pulling that wire loose;  disconnecting that wire revealed the problem.  

If you have problems on multiple circuits in your house, it's especially concerning, you may have a problem with your electrical service (lost neutral or other problem).  The problem may also involve your neighbors.  

It's good that the main power is off, but that doesn't necessarily make everything safe in the house.  I'd call the electric company immediately and consider it an emergency.  Call an electrician too, as soon as possible.  

edit:  The original poster followed up that an electrician came and fixed the issue.  With that resolved, incidental contact with the ground wire where it's exposed in the closet isn't dangerous.  Disconnecting the ground wire from the pipe shouldn't be dangerous, but it can't be assumed to be safe, because there may be an undiscovered issue waiting to bite you.