At least 175 people have been executed in Saudi Arabia in the past year after unfair trials lacking basic safeguards, Amnesty International says.
A new report alleges the Gulf kingdom's "faulty justice system facilitates judicial executions on a mass scale".
In many cases, it adds, defendants are denied access to a lawyer and in some they are convicted on the basis of "confessions" obtained under torture.
Those killed include juvenile offenders and people with mental disabilities.
Saudi Arabia adheres to a strict interpretation of Islamic law, or Sharia, and applies the death penalty to a number of crimes not considered the "most serious" under international norms. They include drug offences, apostasy, heresy and witchcraft.
Amnesty's report says that between January 1985 and June 2015, at least 2,208 people were executed in Saudi Arabia.
Almost half of those put to death were foreign nationals, many of whom the human rights group says were denied adequate translation assistance at trial and were made to sign documents - including confessions - which they did not understand.
In the first six months of 2015 at least 102 people were executed - compared with 90 in all of 2014 - an average of more than one execution every two days. Most were beheaded, but some were shot by firing squad.
The report says that Saudi Arabia's Sharia-based justice system lacks a criminal code, leaving definitions of crimes and punishments vague and widely open to interpretation.
Judges are also given the power to use their discretion in sentencing, "leading to vast discrepancies and in some cases arbitrary rulings", it adds.
The report also alleges that death sentences are often imposed after "unfair and summary proceedings which are sometimes held in secret".
In one case from August 2014, two sets of brothers from the same extended family were executed after being convicted of receiving large quantities of hashish.
Amnesty says the men complained that they had been tortured during interrogation, including with beatings and sleep deprivation, in order to extract false confessions.
"Sentencing hundreds of people to death after deeply flawed legal proceedings is utterly shameful," Said Boumedouha, acting director of Amnesty's Middle East and North Africa programme, said in a statement.
"The use of the death penalty is horrendous in all circumstances, and is particularly deplorable when it is arbitrarily applied after blatantly unfair trials."
Saudi Arabia argues that death sentences are carried out in line with Sharia and with the strictest fair trial standards and safeguards in place.