Mumtaz Qadri was hanged at a prison in the garrison city of Rawalpindi near the capital Islamabad, jail official Ijaz Chaudhry said.
Hardline Sunni Tehreek group chief Sarwat Ijaz Qadri, not related to the hanged man, condemned the execution.
“The martyrdom (execution) of Qadri is a black day in the history of Pakistan,” he said in a statement.
He asked his workers to remain peaceful while announcing that a funeral prayer would be held Tuesday in Rawalpindi.
But the country braced for a possible backlash. Educational institutions were closed in Rawalpindi, while the city’s main Murree Road, which links to it Islamabad, was blocked by the protestors.
Dozens of police in riot gear were in the main squares in Rawalpindi, while army soldiers were deployed in key locations in Islamabad.
Extra police and paramilitary troops were deployed across Punjab province, of which Rawalpindi is a part, and in the southern city of Karachi, which is a stronghold of the Sunni Tehreek, police said.
“The situation is under control but fuel stations at some places are closed in the city. We have increased police patrols,” police official Ijaz Panwar told dpa by phone from Karachi.
The Muttahida Qaumi Movement political party, which dominates Karachi, cancelled its workers’ convention, the party said in a statement.
Protesters clashed with police in some parts of the eastern city of Lahore, capital of Punjab. The government-run Metro bus service there was briefly suspended, police spokeswoman Nabila Ghazanfar said.
“We have reports of some minor injuries to protestors but now the situation is normal. Police are on high alert to deal with any disturbance,” she said.
Qadri was hailed as a hero in the conservative Islamic society after he gunned down Salman Taseer, the former governor of Punjab, in 2011.
Taseer, an outspoken critic of Pakistan’s 1980s blasphemy laws, which impose the death penalty in some cases, had met Christian woman Asia Bibi in jail after she was arrested on charges of insulting the prophet Mohammad.
Bibi remains in jail on death row.
An anti-terror court handed Qadri the death penalty, and his appeal was rejected by the Supreme Court and the president of Pakistan.
The execution took place amid tight security around the prison and in the capital.