A prominent Shia cleric was among 47 prisoners executed by Saudi Arabia over the new year period.
Sheikh Nimr al-Nimr, who was a driving force behind the Arab Spring of 2011, was among those killed at 12 different locations around Saudi Arabia for crimes related to terrorism.
All those put to death - by firing squad or by beheading - were Saudi nationals apart from one person from Egypt and another from Chad.
The execution of Sheikh Nimr is expected to deepen discontent among Saudi Arabia's Shiite minority and heighten sectarian tensions across the region, the Associated Press reported.
Kashmiri Shiite Muslims mourn as they hold portraits of Sheikh Nimr al-Nimr
Also killed were a number of prisoners linked to al-Qaeda.
Saudi Arabia's top cleric Grand Mufti Sheikh Abdulaziz Al Sheikh said the executions were carried out in line with Islamic law and the need to safeguard the kingdom's security.
He described the executions as a "mercy to the prisoners" themselves because he said it would save them from committing more evil acts.
Islamic scholars around the world hold vastly different views on the application of the death penalty in Islamic Shariah law. Saudi judges adhere to one of the strictest interpretations, a Sunni Muslim ideology referred to as Wahhabism.
Influential Shiite figures and groups across the region were swift to condemn al-Nimr's execution, with Saudi Arabia's regional rival Iran describing it as "irresponsible."
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Maya Foa, director of the death penalty team at international human rights organisation Reprieve said: “2015 saw Saudi Arabia execute over 150 people, many of them for non-violent offences. Today's appalling news, with nearly 50 executed in a single day, suggests 2016 could be even worse.
“Alarmingly, the Saudi Government is continuing to target those who have called for domestic reform in the kingdom, executing at least four of them today. There are now real concerns that those protesters sentenced to death as children could be next in line to face the swordsman’s blade.
“Saudi Arabia's allies - including the US and UK - must not turn a blind eye to such atrocities and must urgently appeal to the Kingdom to change course.”
According to the Guardian, Saudi Arabia executed at least 157 people in 2015.