Thousands of antigovernment protesters have begun marching from the eastern Pakistani city of Lahore toward the capital, Islamabad.
The protesters are expected to arrive at Islamabad to stage their rallies by early August 15 at the latest.
They will be joined by thousands of other opposition supporters from all over Pakistan.
The demonstrations are headed by cricketer-turned-politician Imran Khan and populist preacher Tahir-ul-Qadri.
Khan and others allege the 2013 elections that brought Prime Minister Nawaz Sharif to power were rigged.
Khan said the protesters will remain in Islamabad until Sharif resigns and fresh elections are called.
Security has been stepped up in Islamabad with thousands of additional troops deployed in the capital.
The rallies were initially banned but authorities eventually relented.
Some entry points to Islamabad were reopened to allow protesters in.