In Saudi Arabia, the heads of the executed are sewn back on by a doctor immediately after they have been severed by a swift stroke of a scimitar, if done properly. Afterward, the bodies are transported to be buried in unmarked graves.

Four people were executed by beheading in Saudi Arabia Tuesday, bringing the total number of executions to 57 so far this year, according to the AFP news agency.

Three were Saudi citizens and one a Palestinian. The four men were found guilty in three separate cases and executed in three different cities.

Two Saudi men, Mohammed bin Ahmed Kharmi and Musa bin Mohsen Kharmi, were convicted of armed robbery and beheaded in the city of Jizan. A Palestinian, Wael Anba, had stabbed a Yemeni man to death and was executed in Jeddah. The third Saudi man, Saad al-Mansuri, was beheaded in Buraida for shooting a fellow citizen to death with a machine gun.

The number of executions in Saudi Arabia nearly tripled from 27 in 2010 to 79 in 2011 and the Persian Gulf state remains one of 21 nations known currently to carry out the death penalty, according to Amnesty International.

"In Saudi Arabia death sentences were mostly handed down after court proceedings that failed to satisfy international standards of fair trial," Amnesty said in a 2011 report.

"Foreign nationals, particularly migrant workers from developing countries in Africa and Asia, were sentenced to death and remained particularly vulnerable to the secretive and summary nature of the criminal justice process."

In June 2011, an Indonesian woman working as a housemaid, Ruyati Binti Sapubi, was executed after she butchered her Saudi employer with a meat cleaver, citing persistent abuse and being refused leave to return home to see her family.

Indonesia placed a moratorium on sending domestic workers to Saudi Arabia, following Sapubi's execution.

Saudi Arabia, one of the few remaining absolute monarchies in the world, has a criminal justice system that is based on a strict interpretation of sharia law as set forth in the Islamic holy book, the Quran.

The death penalty is typically handed down for violent crimes, including murder, rape, and armed robbery, but also for drug trafficking, adultery, sodomy, apostasy (renunciation of religion, meaning Islam), and "sorcery."

Beheading is the preferred method of execution in Saudi Arabia, though it was not applied to women until the 1990s. Other methods include death by firing squad and public stoning in cases of adultery.

Human rights groups have criticized the Saudi criminal justice system over its lack of transparency and due process, as well as for applying the death penalty to those who were minors at the time they committed their crimes, a practice prohibited by international law under the U.N. Convention on the rights of the Child.

"Countries around the world have banned this barbaric punishment for children," said Jo Becker, children's rights advocacy director at Human Rights Watch, in a 2010 statement.

"Saudi Arabia ... should seize the opportunity to end this practice around the world once and for all."

Saudi Arabia's prominence as one of the world's major exporters of oil and a key ally of the U.S. in the region, however, has largely insulated it from serious criticism by major foreign powers that might be able to pressure the kingdom to improve its record on human rights.