Calling the death of Farzana Parveen "totally unacceptable," Sharif said he asked for "immediate action" and a report on the incident by Thursday evening from the chief minister of Punjab province.
Parveen, who was pregnant, was pelted with rocks and struck with sticks, in broad daylight outside a Lahore court, in an "honor killing" by relatives angry she married for love instead of participating in a traditional, family-arranged marriage. Her husband, Muhammad Iqbal, said police and onlookers stood by and did nothing to rescue her.
"They watched Farzana being killed and did nothing," he said.
His comments were disputed by the Lahore police chief, who claimed Parveen was dead by the time police arrived.
Parveen's father later surrendered to police, but other family members escaped without arrest.
At least 869 women in Pakistan were killed in 2013 in "honor killings," the Human Rights Commission of Pakistan reported, although activists say the number is considerably higher and note the conviction rate in cases of sexual and other violence against women is low.