Articles and comments on the popular online encyclopedia showed Turkey "in coordination and aligned" with terrorist groups, the Turkish Ministry of Transport, Maritime Affairs and Communications said.
"It has become part of an information source which is running a smear campaign against Turkey in the international arena," the ministry said.
Turkey warned Wikipedia to remove such content, but the nonprofit encyclopedia refused, the government said.
Once Wikipedia meets Turkey's demands, the access ban will be lifted, it said.
'Access to information is a fundamental right'
Turkey Blocks, a group that monitors censorship, said the crackdown on access to all editions of the site began as of 8 a.m. local time Saturday.
Wikimedia Foundation, a nonprofit organization that supports Wikipedia, said it is "actively working with outside counsel to seek judicial review of the decision affecting access to Wikipedia."
"Wikipedia is a rich and valuable source of neutral, reliable information," the foundation said in a statement. "We are committed to ensuring that Wikipedia remains available to the millions of people who rely on it in Turkey."
The site's founder, Jimmy Wales, said in a tweet, "Access to information is a fundamental right. Turkish people, I will always stand with you and fight for this right."
Wikipedia says on its site that the encyclopedia is "written collaboratively by largely anonymous volunteers who write without pay."
"Anyone with Internet access can write and make changes to Wikipedia articles, except in limited cases where editing is restricted to prevent disruption or vandalism. Users can contribute anonymously, under a pseudonym, or, if they choose to, with their real identity," it says.
Many observers say Turkey -- a member of NATO and a US ally -- has taken a decided authoritarian turn when it comes to the free flow of information.
Thousands of civil employees dismissed
President Recep Tayyip Erdogan's tenure has been marked by instability.
The country has endured an attempted coup, followed by widespread purges and arrests, a longstanding militant Kurdish rebellion, ISIS terror attacks and insecurity generated by the Syrian civil war on its border.
The purges continued Saturday, with the firing of nearly 4,000 civil employees, according to a decree announced on a government website. The decree said soldiers and academic personnel were among the 3,974 state employees dismissed.
Eighteen academic foundations, 14 nongovernmental organizations and 13 health groups were also shuttered due to activities against national security, Anadolu reported.
Another decree announced Saturday banned television matchmaking shows.
Turkish voters this month passed an 18-article constitutional reform package to transform the country's parliamentary system into a powerful executive presidency.
The plan, put forward by the ruling Justice and Development Party, gives Erdogan sweeping and largely unchecked powers.
International election monitors delivered a scathing verdict on the conduct of the referendum, but Erdogan denied his new powers were a move toward dictatorship.