Cairo (dpa) - Saudi Arabia on Saturday came under blistering criticism from the region's Shiites after it executed a top Shiite cleric known for his activism against the Sunni government.

Nimr al-Nimr was among 47 people the Saudi government said it had executed earlier on Saturday after their convictions on terrorism-related charges.

Saudi sources said four of those executed were Shiite.

An Interior Ministry spokesman said the 47 had been found guilty of adopting extremist ideology, causing explosions and killing civilians and security officers, charges that suggest the remainder were probably linked to the Sunni extremist group, al-Qaeda.

Al-Nimr had been condemned to death in 2014 on charges of causing sectarian strife and disobeying the country's ruler.

Iran, Saudi Arabia's regional Shiite rival, slammed his execution as politically and religiously motivated.

"Instead of focusing on [Islamic State] terrorists threatening the region and the whole world, the Saudis execute a prominent figure like al-Nimr," Iranian Foreign Ministry Spokesman Jaber Ansari said, referring to the radical Sunni militia mainly active in Syria and Iraq.

In Iraq, where sectarian tensions have played a key role in the rise of Islamic State, religious authorities from the Sunni minority were among those condemning the execution.

Iraq's Dar al-Ifta, a key Sunni religious body, said it feared the execution "could cast the sons of the Islamic community into new internal conflict."

"The expression of opinions and peaceful opposition are rights protected by divine and international law," Iraq's Shiite Prime Minister Haider al-Abadi wrote on his official Facebook page.

"Their violation will have repercussions for the security and stability of the region's peoples and their social fabric," he warned.

Saudi Arabia hit back at criticism of the executions, saying the Iranian comments were "irresponsible."

"When implementing court rulings, the kingdom does not pay attention to any threats or comments," spokesman for the Saudi Interior Ministry, Mansour al-Turki, said.

Saudi Arabia's top state-appointed cleric, Abdel-Aziz al-Sheikh, also advocated the executions, saying they were "fair and aimed to boost security."

Al-Nimr, a Saudi national, repeatedly demanded increased rights for the Shiite minority, who make up some 15 per cent of the Saudi population.

Toby Matthiesen, author of a recent book about Shiite dissent in the kingdom, wrote on Twitter that the execution of al-Nimr was "the first execution of a political (as opposed to a militant) dissident in decades in Saudi Arabia."

Chris Doyle, a commentator on Middle East affairs and director of the Council for Arab-British Understanding, said the execution was likely to exacerbate sectarian tensions in the region.

The execution of al-Nimr "won't help in Saudi Arabia's image as an anti-Shia state, in the way that it treats the Shia, in the way that it has stoked up sectarian rhetoric," Doyle told dpa, adding that "Iran also stokes up sectarianism ... the other way round."

Saudi Arabia's Shiites complain of discrimination, saying they often struggle to get senior government jobs and benefits available to other citizens.

The government has repeatedly denied the claims and said that al-Nimr had followed "in the footsteps of the devil" in an Interior Ministry statement cited by the official Saudi Press Agency.

"Through his terrorist acts, innocent blood was shed with the goal to shake stability in this country," it said.

Al-Nimr's brother, Mohammed, called for any response to the execution to be peaceful.

"Naturally there will be reactions commensurate with this despicable act, but I repeat that they must not step beyond the bounds of peaceful action and peaceful protest," he told Lebanese broadcaster Al Mayadeen.

Federica Mogherini, the European Union's foreign policy chief, said that al-Nimr's killing specifically had "the potential of inflaming further the sectarian tensions that already bring so much damage to the entire region, with dangerous consequences."

Al-Nimr's backers in Sunni-ruled Bahrain, which has a Shiite majority, staged protests, with people chanting slogans against the Saudi and Bahraini governments and clashing with police, witnesses said.

Al-Nimr, 55, was detained several times. His last arrest, in 2012, during which he was shot in the leg, triggered days of violent protests in his hometown, Qatif, in mostly Shiite eastern Saudi Arabia.

The executions were the first this year in Saudi Arabia, which is believed to have executed 137 people on various offences last year.

Beheading is a common method of execution in Saudi Arabia, which applies a strict interpretation of Islamic law. The country ranks among the top nations for executions carried out.