They have tried to raise awareness of the dangers of capital punishment and tried to mobilise public opinion against this practise.
Clive Stafford Smith is the founder and director of Reprieve, a UK-based legal charity, and has spent 25 years working on behalf of defendants facing the death penalty in the US.
He tells Al Jazeera why he believes capital punishment is a "horrific" act.
It is rather easy, looking back, to identify the beliefs that our ancestors clung to with a fervent faith.
No doubt, 400 years ago, those who burned witches at the stake thought they were righting evil in society.
Four centuries on, the history books are not kind to them. We know the "witches" were innocent, since no coven of witches actually existed. We now recognise that any trial that sent its victim to the stake was derived from a "witch hunt" that served no possible penal purpose.
It is more difficult, perhaps, to identify our modern "flat earth" beliefs - those present day certainties that will look very foolish when viewed from a globe that is self-evidently spherical.
However, I have absolutely no doubt that when the history books are written 100 years hence, the fact that we were killing our fellow human beings in the name of "just punishment" will be viewed with a mixture of bemusement and horror.
When we think of how our ideal society would behave, does anyone imagine it would include ritual executions?
Divergence of opinion
Amongst the large and ever-increasing body of people who oppose the death penalty, there is considerable divergence of opinion.
There are those who believe that it is state-sanctioned murder, pure and simple; those who oppose it on religious grounds (including the Pope); and those who think it is inevitably beset by racial or economic discrimination (how many millionaires does one encounter on Death Row?).
There are those who also recognise its non-existent deterrent value; those who believe that the diverse frailties of human beings guarantee that there will be mistakes; and doubtless many other variations besides.
When I held a purely theoretical opposition to execution, I used to indulge in all these arguments. When - 22 years ago now - I watched my first client being executed, it rather changed my perspective.
Edward Johnson was young, personable, black, and almost certainly innocent. I was a young lawyer then, and had taken on his case close to the end.
I had failed him. As the gas wafted up toward his lungs in the execution chamber at the Mississippi State Penitentiary, I was utterly disgusted.
How could anyone possibly think this a civilised way to deal with any problem? It was not just about how wrong it seemed - it was how utterly pointless and barbaric.
Mad or bad?
I have represented over 300 prisoners facing execution, and I am more interested in those who are not entirely innocent. Only someone who has never taken the time to meet these people can pretend that they are a distillation of the "pure evil" of society.
And only those who do not care to learn the truth can see these people as devoid of humanity. Just as each victim of murder is a unique human being, so is each person charged with the crime.
And each person tells a story.
Reprieve is currently trying to halt the imminent execution of a British man, Akmal Shaikh, in China. Mr Shaikh suffers from bi-polar disorder, just as my father did before him.
The Chinese court did not even know this when Akmal was sentenced to be shot in the back of the head for allegedly smuggling drugs.
He insists he is innocent, but let us assume he is not: only someone who has never had dealings with a floridly psychotic person could possibly believe he should be executed.
Akmal Shaikh is one of many. Mental illness is prevalent monist most prison populations, but is higher still monist those awaiting execution.
Just as our ancestors preferred to believe that strange happenings were caused by witches, so we do not like to accept the reality of mental illness. It is inexplicable, and we are more comfortable saying people are bad, rather than mad.
Indeed, it takes great courage for the victims of crime to recognise that their own suffering has no rational basis.
In 1992, Ricky Langley was sentenced to death for killing six-year-old Jeremy Guillory. Before Ricky was born his parents were involved in a car crash. Two children died, including the six -year-old Oscar Lee.
The mother suffered horrific injuries and was in a full body cast when Ricky was conceived.
Her pregnancy went undetected for five months, during which time she and her foetus had been prescribed powerful drugs and bombarded with x-rays.
The doctors advised an abortion; her husband, a Catholic, strongly objected. Ricky was therefore born to almost inevitable mental illness.
His parents could neither understand nor accept it, and thought it merely odd when he announced at the age of 11 that he was not actually Ricky Langley, but his dead brother Oscar Lee.
Jeremy Guillory's mother, Lorilei, was desperate to understand why her child had been torn from her. In the end, she spent three hours with Ricky, and realised that he was truly insane.
She too was a Catholic, and opposed the death penalty. But gradually she came to believe what society tells us: that the insane should not be sent to prison, but rather to hospital.
Crying out for help
She testified on Ricky's behalf. I asked her whether she felt that the killer of her child had been mentally ill when he did it.
"I think that Ricky Langley has been crying out for help since the day he was born," she said, turning to the jurors. "And for whatever reason, his family, society, the legal system has never listened to him. And as I sit on this chair, I can hear the death cries of my own child, Jeremy; but I can still hear Ricky Langley crying out for help."
The prosecutor said she was an unfit mother for saying that. I said she was one of my heroes. We cannot expect everyone to invest as much compassion in such a tragedy, but we can tell the difference between our ideal society and the dark world that some would have us inhabit.
And Lorelei points the way toward our salvation. In 2008, around the world, 2,390 people were killed by the machinery of the state.
That is probably one person executed for every million crimes that were committed. Did this ritual sacrifice purify our world? Or did it merely prevent us from seeing as clearly as Lorilei Guillory?