Mumtaz Qadri was executed on February 29 for killing a Punjab governor over his call for blasphemy reform in what analysts said was a "key moment" in Pakistan's long battle against religious extremism.
But it has also exposed deep religious divisions in the conservative Muslim country of 200 million.
An estimated 25,000 supporters of the former police bodyguard gathered in Islamabad's twin city Rawalpindi in the afternoon to offer prayers, before turning toward the heavily-barricaded capital, which was patrolled by hundreds of police and paramilitary soldiers.
A military spokesman tweeted late today that the army had been requisitioned to "control" the situation and secure the Red Zone around Parliament, where protesters were gathering.
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The protest had been largely ignored by the Pakistani media, which has increasingly become subject to government-ordered news blackouts designed to prevent unrest from spiralling out of control.
Qadri was working as a bodyguard for Punjab governor Salmaan Taseer when he shot him in 2011 over the governor's call to reform the blasphemy law, which critics say is frequently misused to oppress religious minorities.
Ex-singer Junaid Jamshed, who is now a prominent Sunni evangelist, was leaving Islamabad airport yesterday night when he was set upon by a group of around six men who were waiting to attack him at the exit.
The incident was captured on mobile phone video and has been widely viewed.
Jamshed was forced to flee back into the airport. In a later Facebook posting, he said it was time for the nation to decide it "will not let these religious fanatics prevail amongst us".
"Get hold of the culprits and make an example of them. Enough of this madness in the name of love," wrote Facebook user Karami Elahi.