Pillay is making her first visit to Pakistan since becoming the UN High Commissioner in 2008.
The visit comes at a time when Pakistan's government and the security establishment are facing considerable criticism on the issue of "missing persons" or those detained without charge by intelligence agencies in Balochistan.
After the Supreme Court took suo moto notice of the issue of missing persons and forced security agencies to produce several people who had been detained without charge for years, Prime Minister Yousuf Raza Gilani decided to visit Balochistan capital Quetta to examine allegations that hundreds of people were in the custody of security agencies.
During her four-day visit, Pillay is scheduled to meet Prime Minister, Foreign Minister Hina Khar and other ministers.
She will also hold discussions with the Chief Justice of the Supreme Court, parliamentarians, members of the legal profession and civil society groups.