The Saudi decree equates the Brotherhood, which has long denounced violence, with widely designated terrorist organizations, including Al Qaeda, Hezbollah, the Islamic State of Iraq and Syria and the Syria-based Nusra Front. The inclusion of the Brotherhood appeared to signal the beginning of a Saudi effort to eradicate the group, demonstrating the deepening polarization that is spreading across the region after the Egyptian military's ouster of President Mohamed Morsi, a Brotherhood leader, last summer.
The decree was the Saudi monarchy's latest gesture of support for attempts by the new military-backed government in Egypt to crush the Brotherhood. But it was also a pointed message to a neighboring Persian Gulf state, Qatar, which has provided refuge and support to Egyptian Brotherhood leaders since the takeover. Consistently sympathetic coverage of the Brotherhood by the Qatari-owned news network Al Jazeera has outraged Cairo and the other gulf monarchies.
The Saudi royal family has always viewed the Muslim Brotherhood with apprehension, fearing its rival blend of Islam and politics as well as its avowed embrace of democracy. The Saudi government prefers to align itself with a more puritanical approach to Islam, Salafism, which teaches heavy deference to Muslim rulers. But Brotherhood members living in Saudi Arabia have not usually felt the need to hide their affiliation for fear of arrest.
Since Mr. Morsi was deposed, though, the Saudi monarchy seems to have embarked on an all-out campaign against the group. As the dominant force in the gulf, Saudi Arabia appears to have led a campaign against Qatar over its support of the Islamists. This week, Saudi Arabia, the United Arab Emirates, Bahrain and Egypt withdrew their ambassadors from Doha, Qatar's capital, in protest. Saudi Arabia and other gulf states donated more than $12 billion to the military-backed government in Cairo almost immediately after it removed Mr. Morsi.
The decree's scope remained a puzzle. It was unclear if Saudi Arabia meant to extend the terrorist label to every affiliate or ally of the Brotherhood in the region. That would include the prime ministers of Turkey and Morocco, the leading party in Tunisia, and recognized opposition parties in Jordan and Bahrain. Some analysts asked if the Saudis would jail Islamist public officials from those countries if they visited, say, on a pilgrimage to Mecca.
Saudi Arabia is also a close ally of the Syrian Muslim Brotherhood in their shared battle against President Bashar al-Assad of Syria. And less than two years ago, King Abdullah of Saudi Arabia met in Riyadh with Mr. Morsi on his first foreign trip after his election. The United States and other Western countries do not consider the Brotherhood a terrorist group.
In a statement issued by its London office, the Brotherhood said it was surprised and distressed by the decree. Unlike the militant groups listed, the Brotherhood said it never declared any government to be "infidel" or a legitimate target of violence. "The Brotherhood takes no stance of enmity or confrontation with the state, but rather acts as an adviser or a guider," it said.
"The Muslim Brotherhood also cooperates with all nations politically in order to achieve common goals, such as having a dignified and free life," the group said.
Jamal Khashoggi, a Saudi commentator, noted in an interview with the Saudi-owned news network Al Arabiya that the Brotherhood was the only organization on the terrorist blacklist that was not an armed group.
"What are the activities of the Muslim Brotherhood in the kingdom?" he asked. "These are not so easy to describe, and the authorities will need to answer the question of how they plan to respond to the group."