Mumatz Qadri, 26, told a judge in Karachi he was angered by Gov. Salman Taseer's opposition to the law, which carries a death penalty, and shot him 26 times. Qadri was assigned to the governor's security detail.
Qadri was ordered detained on a 14-day remand pending another hearing Jan. 24.
Qadri's lawyer told the judge he had acted alone and was not linked to any militant group; however, The Nation said authorities reserved the right to question Qadri again under the remand.
The Washington Post said Qadri became an instant celebrity after last week's assassination of Taseer, the governor of Punjab province. In fact, his court appearance took place a day earlier than scheduled to head off any potential trouble outside the courthouse.
A march in Karachi Sunday drew scores of Islamic fundamentalists who declared him a hero.
"Mumtaz Qadri sacrificed himself to protect the sanctity of our prophet, and everyone here is ready to do the same thing," seminary student Abdul Majid told the newspaper Sunday.