Christian pastor Zafar Bhatti was killed and 70-year-old Briton Muhammad Asghar, who has a history of mental illness, was wounded in the attack in Rawalpindi, next to the capital, Islamabad.
Bhatti, who worked to protect the human rights of the country's beleaguered Christian minority, was on trial after an Islamic leader accused him in 2012 of sending text messages derogatory to the Prophet Mohammed's mother.
His family say police investigations show the phone was registered to someone else.
In recent weeks, Bhatti had received death threats in prison from both inmates and guards, his family told Pakistan-based human rights group Life for All. He was being held in the same cell as Asghar.
"This is a barbaric act. There had been threats. The court should have instructed police to ensure Bhatti's safety," said Xavier Williams of Life for All.
"Killing of a person who was falsely accused is mockery of the judicial system. The protectors of the innocent have become the predators."
Asghar, from Edinburgh, was arrested in 2010 and sentenced to death in January after a disgruntled tenant presented letters he had written saying he was a prophet.
Asghar had previously been detained under the mental health act in Britain and diagnosed with paranoid schizophrenia, according to documents his lawyers supplied to Reuters.
His lawyers were barred from attending the last few months of his trial.
The law firm asked not to be identified for fear of being targeted by extremists.
Blasphemy is punishable by death in Muslim-majority Pakistan.
This year has seen a record number of blasphemy accusations, according to an Islamabad-based thinktank, the Center for Research and Security Studies. Many analysts say the claims are increasingly used to settle scores or grab property.
Blasphemy charges are hard to fight because the law does not define what is blasphemous. Presenting the evidence can sometimes itself be considered a fresh infringement.
Those accused of blasphemy are often lynched and lawyers in defending those accused of blasphemy cases have frequently been attacked. Judges have been attacked for dismissing cases and many of the accused face years in jail as their trials drag on.
At least 48 people accused of blasphemy have been extrajudicially killed, including seven in prison or outside court, according to Life for All.
Earlier this year, a prominent human rights lawyer representing an English professor accused of making a blasphemous Facebook post was shot dead after prosecution lawyers had threatened to kill him in front of a judge.
Last week, gunmen shot dead a liberal professor of Islamic law in the southern city of Karachi. The killing followed years of threats from his colleagues and allegations of blasphemy.
Two prominent politicians who suggested reforming the law have been killed — one by his own bodyguard. Another politician who discussed reforming the law on television is now facing blasphemy charges.
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