Human Rights Watch said the four men had no lawyers during their trial and sentencing, and were denied other basic legal rights. The group called on Saudi Arabia to halt all pending executions and retry those remaining on death row.
"The execution of these four migrants, who had been badly beaten and locked up for years without access to lawyers, is a travesty of justice," Sarah Leah Whitson, the Middle East and North Africa director at Human Rights Watch, said in a report.
"International law only allows states to use the death penalty for the most serious crimes and in the most stringent of circumstances — and neither condition was met in this case," Whitson said.
Saudi officials did not respond to a request for comment on the Human Rights Watch report.
Saudi Arabia follows a strict interpretation of Islam that calls for people convicted of murder, drug trafficking, rape and armed robbery to be executed in public with a sword. Beheaded bodies are displayed only when there is a specific court order in cases considered particularly offensive.
The four Sri Lankans were convicted of forming a gang that robbed several companies, threatened accountants and workers with weapons and shot one of them and stole his car, said the Saudi Interior Ministry.
Earlier in February, investigators from the New York-based Human Rights Watch had met and spoken to one of the four, Ranjith Silva. According to the group's report, Silva was apparently unaware of his imminent execution and was hopeful for clemency.
Silva said he and Victor Corea, Sanath Pushpakumara, and Sharmila Sangeeth Kumara, took up armed robberies in early 2004 because their Saudi employer was paying them each only $67 of the $107 a month agreed in their contract — money that barely covered lodgings.
Silva also told the Human Rights Watch he was never advised he could see a lawyer or that he could face the death penalty. The four were not notified of proceedings ahead of time and had no consular assistance.
Silva also said he was not told how to appeal the verdict and never received a copy of the verdict.
"Defendants sentenced to death must, under international law, have a meaningful right to appeal their verdicts, but these men didn't get the most basic safeguards," Whitson said.
According to an Amnesty International report earlier this week, two of the four may even have been unaware that they had been sentenced to death. The group reported that Sharmila Sangeeth Kumara believed he had been sentenced to 15 years in prison.
After the trial, the Sri Lankans managed to contact their embassy from prison, but were told it was too late to appoint a lawyer. The Sri Lankan government said it had appealed to King Abdullah of Saudi Arabia for clemency twice and was now trying to retrieve the bodies.
Saudi officials would not confirm that the body had been displayed but did confirm the four were executed Monday. They also confirmed the court order for the men's bodies to be displayed after the execution.
Amnesty International has said that besides the four Sri Lankans, six foreigners have been executed this year. They include three Pakistanis, two Iraqis and one Nigerian. Seven Saudi Arabians, including one woman, have also been executed. In 2006, 86 men and two women were executed, half of them foreign nationals, the report said.