Pakistan has executed a man who was 15 when he was sentenced to death for murder and who rights groups say was tortured into confessing.

Aftab Bahadur was hanged in Lahore's jail early on Wednesday morning.

Bahadur, a Christian, had been convicted of a double murder in the city in 1992. Campaigners called his execution "shameful".

The death sentence could be passed on 15-year-olds at the time, although the minimum age was raised to 18 in 2000.

"This is a truly shameful day for Pakistan's justice system. Aftab was subjected to almost every injustice conceivable," said Maya Foa, director from international human rights group Reprieve.

"To the last, Pakistan refused even to grant his lawyers the few days needed to present evidence which would have proved his innocence. This is a travesty of justice, and tragedy for all those who knew Aftab," she added.

Reprieve said Bahadur had been convicted on the basis of testimony from two eyewitnesses. They both later retracted their statements, saying they were made under torture.

In an essay written for Reprieve before his death, Bahadur protested his innocence: "I just received my Black Warrant. It says I will be hanged by the neck until dead on Wednesday, 10 June. I am innocent, but I do not know whether that will make any difference.

"I suppose my life experience is different from that of most people, but I doubt there is anything more dreadful than being told that you are going to die, and then sitting in a prison cell just waiting for that moment.

"For many years - since I was just 15 years old - I have been stranded between life and death," he added.

"I hope I do not die on Wednesday, but I have no source of money, so I can only rely on God and on my volunteer lawyers. I have not given up hope, though the night is very dark."

Executions around the world

- Pakistan has executed about 150 people since December 2014, almost all of them this year

- Figures for executions in other countries in 2015 are not yet available

- By the end of 2014, the countries with the highest number of reported executions were: Iran: 289, Saudi Arabia: 90, Iraq: 61, USA: 35, and Sudan: 23

- In 2013 the numbers were: Iran: 369, Iraq: 169, Saudi Arabia: 79, Somalia: 68, USA: 39

- China and North Korea refuse to divulge information on the number of executions that take place within their borders

What is behind Pakistan's dramatic rise in executions?

The hanging comes a day after the execution of a Pakistani man convicted of manslaughter was postponed at the last moment.

It was the fourth time Shafqat Hussain, who was found guilty of kidnapping and killing a child in 2004, had been given a stay of execution.

His lawyers say he was 14 when he was charged, and was tortured into making a confession.

The authorities however say he was 23 when he committed the crime.

The execution was halted after Pakistan's Supreme Court agreed to consider an appeal.

Pakistan is on course to have one of the highest rates of executions in the world. It executed three other men on the same day that Shafqat Hussain was reprieved and hangings are taking place on an almost daily basis at jails around the country.

The Pakistani government scrapped a moratorium on capital punishment in December after a deadly attack on a school in Peshawar in which more than 150 school pupils and teachers were killed by the Taliban.

Pakistan has the world's largest number of death row inmates, with more than 8,000 people reported to be awaiting execution.