"We were in love," he told The Associated Press. He alleged that the woman's family wanted to fleece money from him before marrying her off.
"I simply took her to court and registered a marriage," infuriating the family, he said.
Parveen's father surrendered after the attack and called his daughter's murder an "honor killing," Butt said.
"I killed my daughter as she had insulted all of our family by marrying a man without our consent, and I have no regret over it," Mujahid, the police investigator, quoted the father as saying.
Mujahid said the woman's body was handed over to her husband for burial.
The Human Rights Commission of Pakistan, a private group, said in a report last month that some 869 women were murdered in honor killings in 2013.
But even Pakistanis who have tracked violence against women expressed shock at the brutal and public nature of Tuesday's slaying.
"I have not heard of any such case in which a woman was stoned to death, and the most shameful and worrying thing is that this woman was killed outside a courthouse," said Zia Awan, a prominent lawyer and human rights activist.
He said Pakistanis who commit violence against women are often acquitted or handed light sentences because of poor police work and faulty prosecutions.
"Either the family does not pursue such cases or police don't properly investigate. As a result, the courts either award light sentences to the attackers, or they are acquitted," he said.
Associated Press Writer Munir Ahmed in Islamabad contributed to this report.