Turkey blocked all access inside the country to Wikipedia on Saturday, in the latest online restriction to hit Turkish internet users.
The information and communication technologies authority said it had implemented the ban on the website, without giving an explanation for the move.
Turkish media said the ban was imposed because Wikipedia had failed to remove content promoting terror and 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 5am GMT after an administrative order by the Turkish authorities, said Turkey Blocks, a group that monitors internet restrictions in the country.
Residents in Istanbul were unable to access any pages of Wikipedia on Saturday morning without using a virtual private network (VPN), which can enable internet users to bypass such restrictions.
"The loss of availability is consistent with internet filters used to censor content in the country," Turkey Blocks said.
The move 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.
Turkey’s Hurriyet newspaper said Turkish authorities had been in contact with Wikipedia to press for the removal of content by writers "supporting terror" and claiming that Turkey collaborated with terror groups.
It said the ban was imposed after the site failed to respond to Ankara’s demands.
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 collaborating with extremist fighters in Syria, a claim fiercely rejected by the government.
There was also speculation that Saturday’s move may have been prompted by deeply unflattering updates to president Recep Tayyip Erdogan’s Wikipedia profile after he won the April 16 referendum on enhancing his powers.
Anyone with access to the internet can write or edit articles on the online encyclopaedia.
Pro-government bloggers said that at one point after April 16, Mr Erdogan was 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.
Turkey has become notorious in recent 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 mass protests against the rule of Mr Erdogan, then prime minister.
The government says such measures are always temporary and needed for national security but critics see them as another restriction on civil liberties under Mr 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.
Later on Saturday, Turkey passed two new decrees – one that expelled more than 4,000 civil servants and another that banned television dating programmes.
The country’s Official Gazette, which published the decrees, named thousands of civil servants to be dismissed, including nearly 500 academics and more than 1,000 Turkish military personnel. It said 236 people would also be reinstated to their jobs.
The state of emergency that followed last summer’s coup attempt has allowed the Turkish government to rule by decrees. Since then, more than 47,000 people have been arrested and 100,000 purged for alleged connections to terror organisations.