Lawyers for the US state of Arkansas have vowed to overturn court orders preventing them from beginning an unprecedented series of executions this week.

A series of legal challenges has blocked the southern state's plans to execute eight men by lethal injection in the space of 11 days.

A judge ruled that one of the drugs used might expose the prisoners to pain before their death - in violation of the US constitution's protection against cruel and unusual punishment.

Arkansas' stock of the drug - midazolam - is due to expire at the end of the month, prompting the rush to carry out so many executions in such a short time.

The state's attorney general has appealed against the court's decision.

Leslie Rutledge said: "We do have a number of pieces of litigation that we are working (on).

"Attorneys are working around the clock and committed to upholding and defending the rule of law, seeing these executions carried out, seeing justice for the families of those victims."

Protests

The planned executions - which would be the most carried out in a such a short time since the US reinstated the death penalty in 1976 - have prompted widespread protests.

Actor Johnny Depp joined one rally, alongside a man who he campaigned to free after 18 years on death row.

When asked what he would say to Arkansas governor Asa Hutchinson, he replied: "I can't, I don't, you know - how do you sleep, man? I don't know. How do you sleep?"

Actor Johnny Depp greets someone as he walks to the podium to speak at a rally opposing Arkansas' upcoming executions. Image:  Stephen B. Thornton/AP/Press Association Images

What is midazolam?

Midazolam has a controversial history, having been linked to a number of botched executions where an inmate, improperly sedated to the effects of the other drugs in their lethal injection, took nearly an hour to die.

The last time two midazolam-using executions were due to be carried out on the same day, in Oklahoma in 2014, the first went so badly that the second was called off.

Several states have been barred by federal courts from injecting the chemical compound, including Mississippi and Ohio, though the Supreme Court Justices upheld the right to use the drug in a 2015 case in a 5-4 decision.

Maya Foa, director of the anti-death penalty campaigners Reprieve, told Sky News: "The drugs slated for use in lethal injection cocktails across the US are simply medicines, designed to save and improve lives and (the) health of patients and (are) being misused in lethal fashion.

"So it is no surprise that the healthcare industry doesn't want to see medicines used in terrible executions," alluding to the legal challenges targeting the manufacturers of the lethal injections drugs.

The maker of midazolam actually no longer sells their drug to states for the purposes of execution.

Death sentences and executions have declined in recent years but voters in a number of states, including California, have opted to keep capital punishment.

Arkansas has not executed any of its convicts since 2005. Six states have abolished the death penalty in the last decade.

The story of the Arkansas eight - Bruce Earl Ward, Don William Davis, Stacey E Johnson, Ledell Lee, Jack Harold Jones Jr, Marcel Williams, Jason F McGehee and Kenneth Williams - is now a key strand of the broader death penalty debate.

For James Phillips it is straightforward. His wife Mary was raped and murdered by Jones.

"I've been waiting for justice for nearly 22 years and that's all I'm after," he said.