They are heeding to separate calls by two leading figures, Imran Khan of the Tehrik-e-Insaf (PTI) party, and an influential religious leader, Tahir ul-Qadri, to converge on the capital to mount pressure on the prime minister.
The rallies on Thursday are seen as the strongest challenge yet to the government of Prime Minister Nawaz Sharif, just a year after he took office in the first democratic transfer of power in a country long plagued by military coups.
The protesters set out on Thursday morning from the city of Lahore in cars, trucks and buses, while others walked or drove motorcycles as they embarked on the 300km long journey to Islamabad, the AP news agency reported.
Police official Mohammad Mahmood said there were about 5,000 protesters on the march, and more were expected to join along the way.
The protest comes after days of escalating political tensions in Pakistan. Khan, the leader of the PTI, the third largest party in the parlilament, is demanding a fresh general election, alleging that Sharif’s Pakistan Muslim League-Nawaz (PML-N) party rigged the 2013 polls.
Khan’s PTI won 34 seats in the National Assembly in that election, compared to the PML-N’s 189.
Qadri, a Canada-based religious leader and a former member of parliament, runs a large international network of religious schools. He says Sharif’s government is corrupt.
Al Jazeera’s Asad Hashim, reporting from Islamabad, said the marches organised by Qadri and Khan were separate, despite both starting from Lahore.
“Thousands are present at both marches, although both are moving at a snail’s pace, It has taken, for example, about four hours for Khan’s convoy to cover about 4km,” he said.
Thousands of Khan’s supporters from the city of Peshawar are also gathered in that city, and are due to make their way towards the capital Islamabad in the coming hours.
The security situation across the march routes and in Lahore and Islamabad can best be summed up as peaceful, but tense. There have been no fresh arrests today, although thousands of police remain deployed.
Roadblocks have been removed from the paths of the convoys, but it is unclear whether this is the case through out the intended route of the convoys, Hashim added.
Clashes between police and protesters at previous rally organised by Qadri in Lahore on June 17 led to the deaths of at least four people.