RIYADH, Nov 5 (Reuters) - One of Saudi Arabia's most prominent businessmen and a former finance minister are among dozens detained in a probe by a new anti-corruption body, a senior Saudi official said on Sunday.

The official, who declined to be identified under briefing rules, said billionaire Prince Alwaleed bin Talal, who owns investment firm Kingdom Holding, and former finance minister Ibrahim al-Assaf had been detained.

Neither man nor Kingdom Holding officials could be reached immediately for comment. Aside from being one of the largest shareholders in Citigroup, Kingdom owns stakes in Rupert Murdoch's News Corp and microblogging site Twitter.

Saudi King Salman announced late on Saturday the creation of a new anti-corruption committee chaired by his son Crown Prince Mohammed bin Salman. Saudi-owned Al Arabiya television said the body had detained 11 princes, four current ministers and tens of former ministers. The crown prince is spearheading an ambitious economic reform programme that aims to draw more foreign and private sector investment into the kingdom, which is the world's top oil exporter and the most powerful country in the Gulf Arab region.

King Salman also appointed two new ministers on Saturday to key security and economic posts, removing one of the royal family's most prominent members as head of the National Guard.

Prince Miteb bin Abdullah was replaced as minister of the National Guard by Khaled bin Ayyaf, while Economy Minister Adel Fakieh was removed in favour of his deputy Mohammed al-Tuwaijri, according to a royal decree carried by state-run media.

Prince Miteb, the preferred son of the late King Abdullah, was once thought to be a leading contender for the throne before the unexpected rise of Prince Mohammed two years ago.

The Cabinet reshuffle helped to consolidate Prince Mohammed's control of the kingdom's security institutions, which had long been headed by separate powerful branches of the ruling family.

Prince Mohammed, who has pledged to go after graft at the highest levels, will head the new anti-corruption body, which was given broad powers to investigate cases, issue arrest warrants and travel restrictions, and freeze assets.

"The homeland will not exist unless corruption is uprooted and the corrupt are held accountable," the royal decree said. (Reporting by Katie Paul, Reem Shamseddine, and Stephen Kalin, and Rania El Gamal in Dubai; Writing by Sylvia Westall; Editing by Andrew Torchia and Paul Tait)