Thousands of opposition activists in Pakistan are heading towards the capital, Islamabad, demanding that the prime minister step down.

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 are seen as the strongest challenge yet to the government of Nawaz Sharif, just a year after he took office in the first democratic transfer of power in a country long plagued by military coups.

As of Friday morning, Qadri's convoy was about 150km from Islamabad, and Khan's 75km further away - both with a few thousand protesters in tow.

The protesters set out on Thursday 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 protest comes after days of escalating political tensions in Pakistan. Khan, the leader of the PTI, the third-largest party in parliament, 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.

Separate protests

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.

"It is unclear at this point if there will be a combined sit-in when they arrive in Islamabad. Both sides are remaining firm on their demand that the government resign, although the PTI and PAT [Qadri's Pakistan Awami Tehreek movement] differ on what happens after that point," he said.

He added that police have been heavily deployed in the capital and roadblocks have been set up near the designated route of the marches.

Thousands of Khan's supporters from the city of Peshawar were also gathered in that city, and were due to make their way towards the capital Islamabad.

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.