Internet traffic for YouTube took a blow in Turkey on Friday, a day after the country's government implemented a blackout before local elections.
Turkey's share of YouTube's global traffic fell by as much as two thirds on Friday afternoon, compared with its levels at the same time on prior days, according topreliminary statistics published by YouTube parent Google Inc. GOOG +0.53% The data indicate that, while not completely effective, the government measure successfully kept out a large number of the site's millions of Turkish visitors.
"This is obviously very disappointing for the many people and businesses in Turkey who use YouTube to share and access information, and we hope that YouTube will be back up in the very near future," a Google spokesman said.
Turkish Prime Minister Recep Tayyip Erdogan said in an interview late Thursday with Turkish broadcaster ATV: "We are determined on this subject. We will not leave this nation at the mercy of YouTube and Facebook. FB -1.57% We will take the necessary steps in the strongest way."

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The sharp decline underscores a delicate dance for the Google video site. YouTube has become a popular form of entertainment and communication in parts of the world with heavy government control over media—from helping spawn the Arab Spring to rebroadcasting an allegedly blocked Ukrainian TV channel. But the site increasingly must weigh that popularity against orders to remove content those governments say is illegal.
That tension is particularly high in Turkey. YouTube is the country's third-most popular website, with 19 million visitors in December according to trade organization IAB Turkiye. But Turkey also makes frequent requests to remove content—with more than 1,500 to Google in the first half of 2013—nearly triple any other country, Google said.
YouTube's popularity has helped put it at the center of a power struggle, after the government of Mr. Erdogan was hit by a corruption probe last year. Anonymous accounts on YouTube have since mid-February regularly leaked phone conversations and other recordings that purport to connect Mr. Erdogan to the investigation, allegations he denies.
But Mr. Erdogan has also used the site to his advantage ahead of local elections on Sunday. At rallies, he has lambasted it, along with social media cousins Twitter Inc.TWTR +2.12% and Facebook Inc., as part of a foreign conspiracy. Last week, beforeblocking Twitter, he called the microblogging site a "menace to society."
Given how easy it is for opposition voters to circumvent the blackouts using software known as virtual private networks, some observers suggest that the blockages are more an electioneering move aimed at rallying supporters than a direct attempt at censorship.
"They are not trying to block social media; they are trying to taint it," said Zeynep Tufekcy, an assistant professor at the School of Information at the University of North Carolina. "Circumventing these blocks is easy and legal, and they know it will happen."
It is unclear how much money is at stake for Google in the Turkey blockage. YouTube makes just over half of its net advertising revenue outside the U.S., according to research firm eMarketer. But the entire Turkish video-display advertising segment in the country was only around $20 million in 2012, the latest year for which data is available from IAB Turkiye.
Google often regularly pushes back against government requests to remove content, current and former executives say. The company says it complied world-wide with only of 36% of requests to remove content in the first half of 2013. Despite the threat of a blackout, the company declined in recent weeks to remove a handful of videos in Turkey, because it believed orders to remove them weren't legally valid, people familiar with the matter said last week.
The showdown between activists and governments often happens without website owners' cooperation. In some countries, regimes can use filtering technology to monitor Web traffic and black out individual videos or Web pages, rather than entire websites. Turkey, however, hasn't yet fully rolled out such technology and had to resort to a "crude" blackout, one Western technology executive said
"But that may change," the executive said.
—Fercan Yalinkilic, Alistair Barr and Rory Jones contributed to this article.
Write to Sam Schechner at sam.schechner@wsj.com