Tens of thousands of Islamists rallied alongside opposition supporters in Pakistan's capital Friday, as the firebrand cleric leading anti-government protests called on Prime Minister Imran Khan to step down within 48 hours.

The so-called "Freedom March" is being led by Khan's long-time rival, Maulana Fazlur Rehman, who heads one of the country's largest Islamist parties - the Jamiat Ulema-e-Islam (JUI-F).

As the first day of demonstrations stretched into the night, Rehman focused his ire on the country's powerful military, which he and other opposition figures have accused of helping former cricket star Khan win last year's election.

"They [the military] have not come to power on the public's mandate but on someone else's direction... they won't work for the public rather they will only please their selectors," Rehman told his supporters from a makeshift stage.

The first day of the protest remained peaceful even as some in the crowd called for moving the rally closer to parliament, just hours after several marches from across Pakistan converged on Islamabad.