Saudi Arabia is one of two nations in the Middle East that maintains a force of Islamic warriors. The largest such force is the Iranian Basij. On paper, the Basij has over 2,000 battalions, each, on paper, with some 500 troops. Although equipped almost exclusively with small arms, most Basij battalions have apparently been reasonably well-trained and some select units may be comparable to regular light infantry. In addition to the "active" Basij, numbering on paper about 1,250,000 men and women, there are supposedly some 2 million more inactive militia members, though they are usually older personnel, and often lack equipment. In practice, there are believed to be about half a million Basij that are armed and available for action.
About a third of the Basij are well enough trained and led to be ready for active service in days. It is these loyal militiamen that the government will use to face down any popular insurrection. The Basij are largely from the countryside, this will allow the government to concentrate military and police units in urban areas, where most of the political reformers are. Several free elections over the past decade have shown that the Islamic conservatives to control the government, only have the support of about 10-20 million Iranians. So, in effect, the Basij is largely composed of that minority of the population that supports the Islamic conservatives, and are willing to carry, and presumably use, a gun to keep their boys in power.
The Saudi force is the National Guard, whose active duty strength is only 75,000. These are organized into eight brigades (three mechanized and five infantry, for a total of 32 battalions.) While much smaller than the Basij, the National Guard is better armed and trained. The National Guard also has a longer, and more illustrious history than the Basij. The National Guard were originally, a century ago, the Ikwhan. These were truly holy warriors, being Bedouin warriors dedicated to the strict Wahhabi form of Islam, and killing enemies of Islam. The founder of Saudi Arabia used the Ikwhan as his shock troops. In the 1930s, the new kingdom of Saudi Arabia had to crack down on the Ikwhan , who were continuing to raid outside the kingdom, since there were no more enemies inside the kingdom to fight. The Ikwhan was shut down, and some of its members helped form the National Guard.
The main purpose of the National Guard, as well as the Basij, was to protect the rulers of their respective countries from the national armed forces. The Saudi army is about the same size as the National Guard, but armed with heavier weapons. Setting up a force to watch the armed forces is a common feature of Middle Eastern governments. Saddam Hussein had his Republican Guard, and other nations have a force of very loyal, well paid and armed troops to protect the boss from his own armed forces. The National Guard is better prepared to this, as the Guardsmen have about a thousand wheeled armored vehicles and some artillery. Nearly all the National Guardsmen troops are Bedouins, usually from tribes that have been historical allies with the al Saud family. The king considers the Guardsmen his boys, and takes good care of them. If a Saudi needs a favor from the king, he's much more likely to get it if he is, or was a National Guardsman.
These "palace guard" type forces are also used to keep wayward civilians in line. The counter-terrorism operations against al Qaeda in Saudi Arabia saw widespread use of the National Guard. Similarly, popular discontent against the Islamic government of Iran is kept under control by the Basij.