It began with a audience members asking Pakistani parliamentary candidate Jibran Nasir to declare his religion at a public forum, a request he politely refused on the grounds it is irrelevant.

Later, the mobs started showed up. In the past week, three of Nasir's public meetings in Karachi, Pakistan's largest city, have been disrupted by religious hardliners - shouting out and shoving his supporters.

Nasir, a human rights lawyer, is a rare breed in Pakistan: an avowedly secular, progressive candidate in the overwhelmingly Muslim country of 208 million, where most parties appeal to an Islamic vote bank.

His independent candidacy is in stark contrast to the prevailing mood of Pakistan's campaign, which has seen the rise of new ultra-right Islamist parties campaigning on protecting the draconian blasphemy laws and denouncing the persecuted Ahmadi sect.

Liberal and secular-minded Pakistanis say the sheer number of religious party candidates, combined with their ultra-conservative rhetoric, has already shifted the agenda in their direction.

Nasir aims to challenge the prevalent extremist discourse.

"Our campaign is bigger than just my win... What it is symbolising for the people is a change in narrative," he told Reuters.

"That challenging extremists is possible ... not using religion to get votes is possible."

Since launching his campaign, a number of clerics in Karachi have branded Nasir a blasphemer from the pulpit, a dangerous accusation in Pakistan that the can lead to the death penalty, and labelled him an Ahmadi, a sect they consider heretics.

On Saturday, a mob of people chanting slogans associated with the new ultra-Islamist Tehreek-e-Labaik Pakistan party, which campaigns under the "death to blasphemers" rallying cry, descended on his public meeting, crowding around Nasir and intimidating his supporters.

"I believe it's being done in a very organised manner," Nasir said.

Labaik representatives could not be reached for comment.

"Our politics should be free from compromise, fear, and discrimination," Nasir said.

Nasir gained nationwide prominence in 2014 after staging protests against the Red Mosque, the Islamabad centre of a militant network with links to Pakistani Taliban strongholds in the northwest and in neighbouring Afghanistan.

The mosque was the site of a military standoff in 2007, but within two years its chief cleric was freed from detention and was once again calling for strict Islamic rule across Pakistan.

The mosque campaign earned Nasir a phone call and death threats from a high ranking Taliban commander.

"We are doing it for the millions of Pakistanis ... who right now are forced to choose amongst the lesser of evils, who are willing to embrace different political leaders with all their biases and religious bigotry," he said.

"Every day I am carrying on with my mission, they are making a fool of themselves."