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Turkey's Information and Communication Technologies Authority (BTK) said it had implemented the ban on wikipedia.org, without making clear the reason for the move.

Turkish media said that the ban was imposed because Wikipedia had failed to remove content 'promoting terror' and also linking Turkey with terror groups.

There was no indication as to when the ban might be removed, with a formal court order expected to follow in the next days.

A block affecting all language editions of the website in Turkey was detected from 0500 GMT after an administrative order by the Turkish authorities, the Turkey Blocks monitoring group, which watches internet restrictions in the country, said in a statement.

Residents in Istanbul were on Saturday morning unable to access any pages of Wikipedia without using a Virtual Private Network (VPN), AFP correspondents said.

"The loss of availability is consistent with internet filters used to censor content in the country," Turkey Blocks said.

The BTK confirmed the ban in a statement but gave no details.

"After technical analysis and legal consideration based on the Law Nr. 5651, an administrative measure has been taken for this website wikipedia.org," it said.

No reason was given for the order to block Wikipedia. Other websites, including leading social media, appeared to be working normally.

- 'Temporary security measures' -

No reason was given for the order to block Wikipedia. Other websites, including leading social media, appeared to be working normally.

But the Hurriyet daily said Turkish authorities had been in contact with Wikipedia to press for the removal of content by writers "supporting terror" and also claiming Turkey collaborated with terror groups.

It said the site had failed to respond to the demands and, as a result, the ban was imposed.

No further details were given but Turkey has always taken a hard line against what it calls "terror propaganda" in favour of the outlawed Kurdistan Workers Party (PKK).

Critics of Turkey have also accused it on occasion of even collaborating with extremists in Syria, a claim fiercely rejected by the government.

Turkey Blocks said the restriction was in place with multiple Internet Service Providers (ISPs).

Turkey has become notorious over the last years for temporarily blocking access to popular sites, including Facebook and Twitter, in the wake of major events such as mass protests or terror attacks.

In March 2014, YouTube was banned for several months in Turkey after the site was used to broadcast purported footage of a security meeting on Syria.

In the summer of 2013, severe restrictions were imposed on social media during the mass protests against the rule of President Recep Tayyip Erdogan, then premier.

Savvy internet users frequently resort to the use of VPNs to get around these bans although there have been complaints that the use of VPNs has now also started to be blocked.

The government says such measures are always temporary and needed for national security but critics see them as another restriction on civil liberties under Erdogan.

In November 2016, Turkey imposed major temporary restrictions lasting several hours on messaging service WhatsApp as well as Twitter, Facebook, YouTube and other sites following the controversial arrests of pro-Kurdish MPs.

Prime Minister Binali Yildirim then acknowledged that "from time to time for security reasons we can use such measures... These are temporary measures. Once the danger is passed, everything returns to normal."

The move to block Wikipedia caused an uproar on social media in Turkey with users angrily denouncing the decision to restrict access to one of the world's most popular websites.

There had also been speculation the decision may have been prompted by deeply unflattering updates by critical users to Erdogan's Wikipedia profile after he won the April 16 referendum on enhancing his powers.

Pro-government bloggers said at one point after April 16 Erdogan had been described as a "dictator" on his main Wikipedia profile.

The government insists that the new presidential system -- largely due to come into force in 2019 -- will improve efficiency, but critics fear it will lead to one-man rule.