The Anonymous hacker collective has knocked out several Saudi Arabian government websites, in a campaign to pressure Riyadh to release activist Ali al-Nimr.

Al-Nimr, a member of Saudi Arabia's Shia minority, was sentenced to be beheaded and crucified for taking part in 2012 anti-government protests. His death sentence was confirmed by a Saudi court last week, prompting international outcry given the already controversial decision to appoint Saudi Arabia to chair the UN Human Rights Council.

Rights groups say al-Nimr was forced to sign a confession under torture, which led to his conviction for a range of crimes including terrorism charges. They also note that under the UN Convention on the Rights of the Child - to which Riyadh is a signatory - it is illegal to execute a child under the age of 18 at the time the crime was committed. Al-Nimr was just 17 when he was arrested.

As of Sunday morning, hackers appear to have taken down a long list of Saudi government websites, including the Ministry of Justice, Ministry of Civil Service, Saudi Airlines and even PSATRI, the Gulf Kingdom's military technological center.

Noting that Saudi King Salman must still approve the death sentence for it to come into effect, hackers threatened to expand its cyber attacks if he did not commute it.

The cyber offensive has been given the hashtag #OpNimr.

"13 Judges have already approved the death sentence of Ali Mohammed al-Nimr meaning only King Salman bin Abdulaziz Al Saud has to approve it. We cannot and will not allow this to happen. The ministry of justice was taken offline a few days ago and we will continue to do this to other government websites," they said in a video published Saturday.