Nearly 20 members of the woman's family, including her father and brothers, attacked her and her husband with batons and bricks in broad daylight before a crowd of onlookers in front of the high court of Lahore, the police investigator Rana Mujahid said.
Hundreds of women are murdered every year in Muslim-majority Pakistanin so-called " honour killings" – carried out by husbands or relatives as a punishment for alleged adultery or other illicit sexual behaviour – but public stoning is extremely rare.
Mujahid said the woman's father has been arrested for murder and that police were working to apprehend all those who participated in the "heinous crime".
Another police officer, Naseem Butt, identified the slain woman as Farzana Parveen, 25, and said she had married Mohammad Iqbal against her family's wishes after being engaged to him for years.
Her father, Mohammad Azeem, had filed an abduction case against Iqbal, which the couple was contesting, her lawyer Mustafa Kharal said. He confirmed that she was three months pregnant.
Arranged marriages are the norm among conservative Pakistanis, who view marriage for love as a transgression.
The Human Rights Commission of Pakistan, a private group, said in a report last month that some 869 women were murdered in "honour killings" in 2013.
But even Pakistanis who have tracked violence against women expressed shock at the brutal and public nature of Tuesday's killing.
"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 in front of a court," 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.
Parveen's relatives had waited outside the court, which is located on a main downtown thoroughfare. As the couple walked up to the main gate, the family members fired shots in the air and tried to snatch her from Iqbal, her lawyer said.
When she resisted, her father, brothers and other relatives started beating her, eventually pelting her with bricks from a nearby construction site, Iqbal said.
Iqbal, 45, said he started seeing Parveen after the death of his first wife, with whom he had five children.
"We were in love," he told the Associated Press. He alleged that the woman's family wanted to swindle 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 incident and called the murder an "honour 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 had been handed over to her husband for burial.