Saudi social media users interpret the rare execution of an Al Saud prince as a sign of equality under Islamic law, approval likely to reassure Riyadh's absolute rulers as large spending cuts test the kingdom's welfare-based social contract.

Prince Turki bin Saud al-Kabir was found guilty of shooting dead another Saudi national during a brawl and was executed after a royal order by King Salman, the interior ministry said on Tuesday.

Members of the ruling family, estimated to number several thousand, have only rarely been executed. A prince who assassinated his uncle, King Faisal, was beheaded in 1975 and a princess was shot dead for adultery two years later.

In 2004, a young prince who had gunned down an acquaintance was pardoned by the victim's father at the last minute, after arriving at Riyadh's execution square.

News of the execution, which comes as the government is asking Saudis to accept unprecedented austerity, was welcomed by Saudis online, including royal family members who said King Salman had been "decisive" and "fair".

Khalid al-Saud, an academic and royal family member, wrote on a Twitter account: "This is the law of God Almighty, and this is the approach of our blessed nation."

"A just ruler of the Islamic nation rules by the legitimacy of Islam," tweeted Saudi journalist Nasser bin Fareon.

Social media users shared a clip of a previous speech by King Salman to Saudi officials telling citizens they should not fear suing royal family members if they had suffered injustice.

Mohammed al-Masloukhi, the imam of the Safa mosque in Riyadh, said the family of the victim had declined to accept hundreds of millions of riyals in "blood money".

"God does not differentiate between a merchant and a poor man, nor between a prince and a citizen," he tweeted.

But a Saudi university student from Jeddah, who spoke on condition he be identified only as Bahah, said the ruling was a reaction to people's frustration with apparent impunity enjoyed by royals.

"It had to happen. It's impossible to continue forgiving members of the royal family who commit crimes," he said by telephone.

"There are political consequences for not prosecuting members of the family, (because) the victims can be members of big tribes. There has to be answers."

Saudi officials say the judicial system is impartial.

Prominent commentator Jamal Khashoggi told Reuters Saudis were receptive to the decision. "It's a sign of respect to the sharia (Islamic law). People like that," he said.

While a culture of public respect for government endures in Saudi Arabia, it is one of the most active Arab societies online, and digital platforms are seen as a useful forum where official accountability can be assessed.

Saudi Arabia executed at least 151 people in 2015, the most since 1995, according to Amnesty, making it the third most prolific executioner after China and Iran.