YouTube attracts a monthly audience of 70 million people
The ban was imposed after prosecutors told the court that clips insulting former Turkish leader Mustafa Kemal Ataturk had appeared on the site.
According to Turkish media, there has been a "virtual war" between Greek and Turkish users of the site, with both sides posting insulting videos.
The clip prompting the ban reportedly dubbed Ataturk and Turks homosexuals.
Insulting Ataturk, the founding father of modern Turkey, or "Turkishness" is an offence which can result in a prison sentence.
The offending videos sparked a storm of complaints to YouTube and the clips were removed, but the court order goes further, blocking all access to the site.
Freedom of speech
Turkish visitors to the site are now greeted with a message in English and Turkish reading "Access to www.youtube.com site has been suspended in accordance with decision no: 2007/384 dated 06.03.2007 of Istanbul First Criminal Peace Court".
Paul Doany, the head of Turk Telecom, the country's largest telecoms company, said that they had blocked access to the site as soon as the court order came through.
"We are not in the position of saying that what YouTube did was an insult, that it was right or wrong," Mr Doany told Anatolia news agency. "A court decision was proposed to us, and we are doing what that court decision says."
Mr Doany said that for its part Turk Telecom will continue to enforce the ban as long as the order stands.
The European Union, which Turkey is hoping to join, has long called for an easing of Article 301 - the law which prevents insults to Turkish culture - arguing that the law places severe restrictions on free speech in Turkey.
About 50 writers in the country have been put on trial for allegedly contravening the rule, including Nobel prizewinner Orhan Pamuk, though most cases have eventually been dismissed by the judge.
Flag of Turkey.
A court in Istanbul has issued an order to block all access to YouTube in Turkey, after a series of insults between Greek and Turkish users of the site escalated. The largest internet provider and privatised state company, Türk Telekom, has put the order into effect, while some smaller internet providers still allow access to the site.

Internet users accessing via Turk Telecom get a message in Turkish saying "Access to this site has been denied by court order! ..." and in Turkish and English "Access to www.youtube.com site has been suspended in accordance with decision no: 2007/384 dated 06.03.2007 of Istanbul First Criminal Peace Court."

The row between Turkish and Greek YouTube users started when Greek videos claimed that Atatürk, the founder of the Turkish state, and the Turkish people, were homosexual. There were also profanities on the Turkish flag. A Turkish user responded saying that Greece was the birth place of homosexuality. The dispute received a lot of attention in the Turkish media, which they labelled a "virtual war".

YouTube agreed to take down the offending videos, but nonetheless the prosecutor in Istanbul got a court order, based on charges of insulting Atatürk, which is illegal in Turkey. YouTube responded that it had taken down the videos and was cooperating with the government, adding that "While technology can bring great opportunity and access to information globally, it can also present new and unique cultural challenges."

Article 301 of the Turkish penal code, which makes insulting Turkishness a crime in Turkey, received a lot of attention because it resulted in the prosecution of intellectuals like Literature Nobel Prize Winner Orhan Pamuk and murdered journalist Hrant Dink.

The European Commission has shown concern over the law article, and Turkey has pledged to revise it. Turkey wants to join the European Union but the negotiations have not been without obstacles so far, for example in the field of freedom of expression.