FACTBOX - Prince Nayef one of most powerful Saudi princes

Saudi Interior Minister Prince Nayef bin Abdul-Aziz (C), brother of Saudi King Abdullah in Riyadh, May 23, 2010.

REUTERS - Interior Minister Prince Nayef bin Abdul-Aziz is one of the most powerful players in leading oil exporter Saudi Arabia.

As interior minister and second deputy prime minister, Nayef is considered to be in a strong position to assume leadership of the country in the event of serious health problems afflicting the king and crown prince.

King Abdullah is travelling to the United States on Monday for medical checks, while Crown Prince Sultan is returning from a holiday abroad.

Here are some facts about Prince Nayef, his career, positions and standing in the ruling Al Saud family.

WHO IS PRINCE NAYEF?

* Nayef was born in the western city of Taif around 1934 and is the half-brother of King Abdullah and son of state founder Abdul-Aziz Ibn Saud. He became Riyadh governor at the age of 20.

* He has been interior minister since 1975 and got promoted as second deputy prime minister in 2009 when Crown Prince Sultan was out of the country to convalesce after medical treatment.

* Nayef played a crucial role in dealing with a series of al Qaeda attacks on expatriate housing compounds, oil and industrial facilities inside the kingdom from 2003 to 2006.

* The confrontation strengthened Nayef's pivotal position in the monarchy and helped him extend his authority into foreign policy, religious affairs and the media. Since his promotion he has taken on additional duties by chairing cabinet meetings while Abdullah and Sultan were away.

* His son, Mohammed bin Nayef, is the Saudi counter-terrorism chief.

* Prince Nayef is one of the most conservative forces in the ruling family and is feared by liberals and reformers. Shortly before his promotion last year, Nayef said he saw no need for female lawmakers or to hold elections at all.

* He has repeatedly backed the religious police apparatus who roam streets to make sure that shops and restaurants close for prayers and unrelated men and women do not mix.

WILL NAYEF OPPOSE REFORMS?

* Diplomats say Nayef is unlikely to pursue many social or political reforms if he were to become king one day. They say his ministry was behind the banning of the country's only film festival in summer 2009, a move that shocked liberals.

* While Nayef is not expected to reverse much-needed economic reforms to create jobs, he may slow plans to overhaul the judiciary because it would mean curbing the influence of clerics who command wide powers as judges, diplomats say.

* Such a move would reduce the kingdom's appeal to investors because the country lacks a consistent legal framework. Clerics base rulings on personal interpretation of Islamic law. The same goes for the ongoing reform of state education, which now focuses on religion and does not produce graduates capable of landing jobs in the private sector. Nayef might bow to demands of clerics to stop or at least slow the reform process here.

* A big general problem in getting reforms under way is that clerics and conservatives dominating ministries simply do not implement orders from the top. If Nayef took over they might feel encouraged to block even more plans, Western diplomats say.

* Other diplomats in Riyadh say Nayef will have no choice but to continue with some reforms as the kingdom needs to attract investors to create thousands of jobs for its rising population but overall doubts over the course of reforms remain.

* Nayef raised eyebrows abroad after the Sept. 11, 2001 attacks in the United States when he denied that Saudis were among the hijackers, suggested Jews instead were behind the attacks, and held up cooperation with Western security bodies. But he went on to win plaudits from Western governments for his action to crush al Qaeda cells inside the kingdom after the group launched a violent campaign there in 2003.

* In June he admonished the religious police to treat people with respect, a move described by an official at the interior ministry as showing that Nayef can be flexible.

"He had gained their confidence by praising them much first, and then asked for change. If he criticised the religious police from the start they would not listen to him," the official said.

(Reporting by Ulf Laessing and Asma Alsharif; editing by Mark Heinrich)