Saudi Arabia is under pressure to quickly choose a replacement for the late Crown Prince Nayef, who was heir to the kingdom's throne and its security chief, as it confronts rivalries and turmoil across the Middle East.

While defence minister Prince Salman appears highly likely to be appointed crown prince, possibly when the formal condolence period ends on Tuesday night, it is not clear who will replace Nayef as interior minister.

Nayef, who died on Saturday, oversaw security for 37 years and built a formidable apparatus that crushed al Qaeda inside the kingdom and remains a vital element of the global struggle to foil Islamist militants.

His tough legacy, involving thousands of arrests of suspected militants, an intelligence network that infiltrated Islamist cells, and an intolerant approach to political dissenters, remains intact but will need a new chief.

'My assumption would be that whoever is given the official job as minister of interior, the division of responsibilities in terms of counter-terrorism and broader internal security would continue as is,' said Neil Partrick, a Gulf security expert at the London School of Economics.

Two possible candidates for the security post are Nayef's brother Prince Ahmed, a veteran deputy interior minister who handles broader security issues including policing, and his son, Prince Mohammed bin Nayef, who now handles counter terrorism.

The new crown prince will eventually succeed to power having to deal with significant domestic and external challenges, including a substantial security file.

Saudi Arabia fears an al Qaeda wing in Yemen and sees rival Iran as instigating unrest among its own Shi'ite minority. The Syria conflict also has elements of a proxy war between Tehran and Riyahd.

And while Saudi emerged unscathed from last year's Arab Spring, which has brought ally Egypt to the brink of government by the Muslim Brotherhood, with which Riyadh has an uneasy relationship.

The kingdom's new heir apparent will also face a number of other challenges including long-term joblessness.

Under Saudi law, King Abdullah can wait 30 days before declaring a new heir. But Saudis with close ties to the ruling family say a decision could be made as early as Monday night.

'My sense is that there will not be any profound changes. Continuity will be the overriding theme,' said Asaad al-Shamlan, a political science professor in Riyadh.

Many analysts expect it to be Prince Salman, 76, a half-brother of the 89-year-old Abdullah and a full brother of Nayef. Salman would be likely to continue with cautious social and economic reforms, anlaysts said.

'The thing that Salman's going to have to do is try and get some influence over some of the religious establishment in the country,' said Michael Stephens, researcher at the Qatar-based Royal United Services Institute.

'He's going to have to start commenting on regional security issues as well as foreign policy initiatives,' he added.

Although some sons of Ibn Saud, such as the deputy interior minister, Prince Ahmed, the Riyadh governor, Prince Sattam, and the intelligence chief, Prince Muqrin have long government experience, the family might prefer one of the founder's grandsons.

'We have seen Abdullah make pretty hard-headed decisions, unsentimental decisions, about family jobs. He chose Nayef over eight living princes. He has shown that age and seniority give way to competence and appetite for the job,' said Robert Lacey, author of 'Inside the Kingdom'.

That could point towards Prince Khaled al-Faisal, the governor of Makkah Province and a son of the late King Faisal, or Prince Mohammed bin Nayef, son of the late crown prince and Saudi Arabia's security chief.-Reuters