* Saudi started blocking apps from 2013
* Arab Spring protests in 2011 often organised via Internet
* Lifting ban part of broader reforms to diversify economy
* Saudi says use of apps will be monitored and censored
* Encumbent telecoms operators could lose voice call revenue (Adds Zain CEO comments)
RIYADH, Sept 21 (Reuters) - The Saudi government is lifting a ban on calls made through online apps such as Skype and WhatsApp on Thursday as part of its economic reforms, but a spokesman said it will monitor and censor such calls.
All online voice and video call services - such as Microsoft's Skype, Facebook's WhatsApp and Messenger, and Rakuten's Viber - that satisfy the kingdom's regulatory requirements, were set to become accessible.
Lifting the ban is part of the Saudi government's broader reforms of the economy to help boost businesses and diversify the economy in response to low oil prices.
Adel Abu Hameed, a spokesman for telecoms regulator CITC, said on Arabiya TV on Wednesday that new regulations aimed to protect users' personal information and block content that violated the kingdom's laws.
Asked if the apps could be monitored by the authorities or companies, he said: "Under no circumstances can the user use an application for video or voice calling without monitoring and censorship by the Communications and Information Technology Commission, whether the application is global or local."
It was unclear how the authorities can monitor apps such as WhatsApp, which says its messages are supported by end-to-end encryption, meaning the company cannot read customers' messages even if approached by law enforcement agencies.
Internet communications have become widespread but Saudi Arabia began blocking them from 2013, wary that such services could be used by activists.
The "Arab Spring" mass protests in 2011 were often organised over the Internet, though Gulf Arab states, except the island kingdom of Bahrain, mostly escaped the uprisings.
The decision to lift the blocks could negatively impact Saudi Arabia's three main telecoms operators - Saudi Telecom Co (STC), Etihad Etisalat (Mobily) and Zain Saudi - which earn the bulk of their revenue from international phone calls made by the millions of expatriates living in the kingdom.
Zain Saudi's CEO, Peter Kaliaropoulos, told Reuters some lost income could be recouped through expansion of its own data services.
"The Saudi market has a strong appetite for faster data throughput and higher data use packages," he said in an email. "The opportunity to monetize the extra data usage will partially offset voice revenue losses".
(Reporting by Stephen Kalin, additional reporting by Alexander Cornwell; Editing by Elaine Hardcastle)