Pakistani police investigating the murder of a pregnant woman bludgeoned to death outside a court have arrested four men including her uncle and two cousins, a senior officer said Friday.
Farzana Parveen was killed on Tuesday outside the High Court in the eastern city of Lahore by more than two dozen attackers armed with bricks, including numerous relatives, for marrying against her family's wishes.
The brazen, brutal nature of the killing, in broad daylight in the centre of Pakistan's second largest city, has triggered outrage around the world.
Police were apparently at the scene, but did not intervene.
Prime Minister Nawaz Sharif on Thursday told Punjab Chief Minister Shahbaz Sharif, who is his brother, to take immediate action on the case.
"Special investigation teams set up by police on the directive of the chief minister arrested four more people including an uncle and two cousins of the slain woman and a driver on Thursday night," senior police official Zulfiqar Hameed told AFP on Friday.
The new arrests bring the number held over the killing to five, after Parveen's father was detained at the scene of the attack.
Shahbaz Sharif has ordered police to round up all those involved within 24 hours.
He has also directed that the case should be heard in an anti-terrorism court. This should mean it moves more quickly than in a regular court.
Pakistan's regular courts are notoriously slow, with many cases dragging on for years with little progress.
The country's Supreme Court has demanded a police report on the incident within 48 hours.
The United States branded Parveen's killing as "heinous", with a State Department spokeswoman denouncing "violence that occurs in the name of tradition and honour, such as so-called honour killings".
In a macabre twist to the case, on Thursday Parveen's husband Mohammad Iqbal admitted to AFP that he had strangled his first wife -- but was spared prison because he was forgiven for the act by his son
"I was in love with Farzana and killed my first wife because of this love," Iqbal said.
When she was killed, Parveen was arriving at court to testify in Iqbal's defence after he was accused by her relatives of kidnapping her and forcing her into marriage.
Iqbal, a farmer, said Parveen's family initially agreed to their marriage before turning against it because they were unhappy with the dowry he offered.
Last year, 869 women died in so-called "honour killings", according to the independent Human Rights Commission of Pakistan. 
Conviction rates are very low due to Pakistan's blood-money laws, which allow relatives to forgive perpetrators, who are usually family members in such cases.