Mashayekh will be executed at the same time and then, to fulfil his additional sentence, his body will be displayed to the public in a cruciform position for three days.
The seven were convicted of armed robbery, but one of the men, in an interview yesterday from his prison cell, claimed that the group was unarmed when they stole jewellery from a string of shops in the southern Saudi Arabian city of Abha in 2004 and 2005. He said that confessions to armed robbery, used as the basis of their trial for robbery in January 2006, were beaten out of the men.
Amnesty said that that the sentences were in any case excessive, while Human Rights Watch added that at least two of the group were minors at the time of the crimes, and none was over 20. "Their trial lasted only a few hours, and they were denied any legal representation or appeal," an Amnesty spokesman said.
"Security officers who were present at the trial warned them that if they withdrew their 'confessions' they would be tortured again, and members of their families, including their mothers, would be brought to prison and tortured in front of them.
"We are urging the King to halt immediately the executions, and calling on the the authorities to investigate the seven men's allegations that they were tortured and otherwise ill-treated.
Saudi authorities regularly order beheadings and other forms of death sentence for rape and murder, and while armed robbery can also attract the ultimate penalty it is employed more rarely.
Crucifixion is occasionally ordered as an extra humiliation - and warning - even where the method of initial execution is beheading.
Abha is in the country's far south-west, and more conservative than other parts of the kingdom such as Jeddah. Without a written legal code, judges in Saudi Sharia courts have wide flexibility to impose verdicts and sentences.
The Washington-based Institute of Gulf Affairs, which is also campaigning against the sentences, claims that people from the south of the country are regarded as second-class citizens and regularly receive harsher punishments as a form of deterrence against unrest.
One of the seven men, Nasser al-Qahtani, used a smuggled phone to tell the Associated Press from his cell that he was only 15 at the time of the crime.
"I killed no one," he said. "I didn't have weapons while robbing the store, but the police tortured me, beat me up and threatened to assault my mother to extract confessions that I had a weapon with me while I was only 15. We don't deserve death."
He said the judge at his trial took no notice of the torture claims.
"We showed him the marks of torture and beating, but he didn't listen," he said. "I am talking to you now and my relatives are telling me that the soil is prepared for our executions tomorrow."
In a statement, Eric Goldstein, deputy Middle East director of Human Rights Watch, said: "It will be outrageous if the Saudi authorities go ahead with these executions.
"It is high time for the Saudis to stop executing child offenders and start observing their obligations under international human rights law."