Iran has accused regional rival Saudi Arabia of supporting terrorism after the kingdom said it had executed a top Shi'ite cleric on terrorism charges.
Nimr al-Nimr, 56, was among 47 people executed in Saudi Arabia on January 2, the kingdom's Interior Ministry said.
"The Saudi government supports terrorists and [radical Sunni] extremists, while executing and suppressing critics inside the country," Iran's Foreign Ministry spokesman Hossein Jaber Ansari was quoted as saying by Iranian state media.
Ansari warned Saudi Arabia "will pay a high price for following these policies." The Foreign Ministry also summoned Saudi Arabia's charge d'affaires in Tehran to protest Nimr’s execution, Iranian state television said.
The 47 were convicted of involvement in terrorist acts and inciting violence, the ministry said in a statement.
Nimr was a central figure in Shi'ite protests that erupted in 2011 as part of the Arab Spring in the Sunni-ruled kingdom's east, where the Shi'ite minority complains of marginalization.
Iran previously warned that executing the cleric would "cost Saudi Arabia dearly."
Ayatollah Ahmad Khatami, a member of the Assembly of Experts and a Friday prayer leader, denounced the execution as a "crime" by Saudi Arabia's "infamous regime."
"This…blood will stain the collar of the House of Saud and wipe them from the pages of history," Khatami was quoted as saying on January 2.
Iran’s powerful Islamic Revolutionary Guards Corps (IRGC) said in a statement that the Saudi government will pay for “this shameful act,” which it said was a sign of decay of Saudi rulers.
The Twitter account of Iran’s Supreme Leader Ayatollah Khamenei paid tribute to Nimr calling him a “martyr.”
"Awakening is not suppressible," read the tweet on Khamenei's English-language Twitter account, next to a photograph of Nimr.
A small group of seminary students protested the cleric's execution in front of the Saudi embassy in Tehran on January 2, Iranian domestic media reported.
The cleric's execution also prompted angry reactions in other countries in the region, including in Shi'ite majority Iraq and in Bahrain.
Iraqi lawmaker Muhammad al-Sayhud warned that Nimr's execution was intended to fuel sectarian strife in the region.
"This measure taken by the ruling family [of Saudi Arabia] aims at reigniting the region, provoking sectarian fighting between Sunnis and Shi'a," he told Al-Sumaria TV.
Prominent Iraqi Shi'ite cleric Muqtada al-Sadr called for demonstrations in Persian Gulf countries and in Iraq to protest the execution of Nimr.
"I ask that the Shi'a of Saudi Arabia...show courage in responding even through peaceful demonstrations, and the same for the Shi'a in the Gulf, so as to deter injustice and government terrorism in the future," Sadr said on his website.
In Bahrain, police used teargas against several dozen people protesting Nimr’s execution while carrying his pictures.
Meanwhile, Nimr's brother said the family was shocked by news of the execution but hoped that any reaction would be peaceful.
"We hope that any reactions would be confined to a peaceful framework. No one should have any reaction outside this peaceful framework. Enough bloodshed," Muhammad al-Nimr told Reuters.
He said the cleric was found guilty of seeking "foreign meddling" in the kingdom, "disobeying" the country's rulers, and taking up arms against the security forces.
Hundreds of members of its Shi'ite minority were arrested after the protests during which several policemen were killed in shooting and petrol bomb attacks.
The kingdom also detained thousands of militant Islamists after a series of Al-Qaeda attacks from 2003-06 that killed hundreds, and has convicted hundreds of them.
The ministry said the executions were carried out on January 2 in 12 different areas of the kingdom.
The executions are Saudi Arabia's first in 2016. At least 157 people were put to death last year, a big increase from the 90 people killed in 2014.
With reporting by Reuters, dpa, AFP, AP, and the BBC