Istanbul - Turkey blocked YouTube on Thursday, a week after it banned the social media website Twitter, provoking a backlash of condemnation by Western nations and freedom of speech activists.
The European Union's top official for digital affairs called the move "desperate and depressing".
"We in Europe stand for an open internet and free expression on it," EU Digital Agenda Commissioner Neelie Kroes said in a statement.
The move came ahead of local elections on Sunday. According to Turkish state news agency Anadolu, the block was requested by the foreign ministry, which cited a "first degree" national security threat.
Prime Minister Recep Tayyip Erdogan has been waging a campaign against social media sites, which have been used in Turkey to organise anti-government protests and to share corruption allegations targeting top government officials.
The latest leaked audio recording to circulate on social media - which cannot be independently verified - alleges that the country's intelligence chief and foreign minister discussed intervening militarily in Syria.
Those behind the leaked recording about the Syria action have been declared "enemies of the state" and will receive "the heaviest punishment possible", according to a foreign ministry statement.
Those trying to access the popular video-sharing platform using a Turkish internet address on Thursday were met by a message reading: "Access has been blocked by Telecommunication Communication Presidency." Erdogan, premier since 2003, has also lashed out at Facebook.
Use of proxy networks (VPNs) and the Tor anonymising network has been on a steady increase since Twitter was blocked last Friday.
Data from Tor showed the use of the free service had more than doubled, surpassing 60 000 users. VPN usage has also skyrocketed, with millions of programme downloads this past week.
Some companies abroad have started offering free services to Turkish users to help them circumvent the ban. Despite such attempts, the ban is having an affect. Some estimates say there have been 40% fewer Turkish language tweets since the Twitter ban came into affect.
Twitter has about 10 million users in Turkey.
A United States state department official compared the Twitter ban to "21st century book burning", while the office of the UN High Commissioner for Human Rights in Geneva said the blockade likely contravenes Turkey's obligations under international law.
YouTube, which is now owned by Google, was blocked for about two years until 2010 in Turkey. Tens of thousands of websites are still believed to be blocked for political reasons and other sensitivities. Turkey also has the largest number of jailed journalists in the world.
A recent law was passed giving the government more power to block websites and track users' browsing habits. Twitter has joined local activists and lawyers in Turkey fighting the block through the courts.
An Ankara court on Wednesday ordered a stay of execution on the ban, but the government has not yet acted on the judicial verdict.
"Political speech is among the most important speech, especially when it concerns possible government corruption," Twitter said in a recent statement, noting that part of its argument with the government is about taking down an account that has made corruption allegations against a minister.
Activists, headed by Yaman Akdeniz, a law professor at Istanbul Bilgi University, have turned to the European Court of Human Rights to request an immediate ruling on the ban, arguing that it could interfere with the elections.
Erdogan's Justice and Development Party's (AKP) is expected to be confirmed as the most popular party in Sunday's elections, though recent graft scandals could hurt its polling.