From the U.S. State Department:

The Department of State warns U.S. citizens to defer all non-essential travel to Pakistan. This Travel Warning replaces the Travel Warning dated August 27, 2012, to remind U.S. citizens of ongoing security concerns in Pakistan.

Protests have taken place across Pakistan against the United States, International Security Assistance Force (ISAF), and NATO. There have also been widespread demonstrations and large political rallies condemning drone strikes, Pakistan's ongoing energy crisis, and Pakistan's July 3, 2012, decision to reopen NATO transit routes to Afghanistan. These protests and demonstrations are likely to continue. U.S. citizens in Pakistan are strongly urged to avoid protests and large gatherings. ...

U.S. State Department Warning