Two convoys are taking thousands of anti-government protesters from the Pakistani city of Lahore to Islamabad, the capital, amid tight security.
The protesters are separately led by opposition politician Imran Khan and anti-government cleric Tahirul Qadri.
They want PM Nawaz Sharif to resign and have promised a change of government.
Both camps are angry about the sinking economy, growing militancy, and failure to deliver core services such as a steady electricity supply.
Mr Khan has also accused Mr Sharif of failing to probe fraud in last year's polls.
The two leaders and their followers are expected to travel 350km (218 miles) with cars, trucks, motorbikes and jeeps, says the BBC's M Ilyas Khan.
However, the convoys were making slow progress and had yet to leave Lahore as night fell on Thursday. A Reuters reporter estimated that a total of 17,000 people had turned up in support of Mr Khan and Mr Qadri.
The leaders are expected to arrive at Islamabad to stage their rallies by early Friday at the latest.
The government says the protesters are trying to derail democracy. But it also announced on Tuesday that a Supreme Court commission would investigate allegations of rigging in the 2013 poll.
This was dismissed by Mr Khan, who said he would only call off the march if Mr Sharif resigned.
