The Organization for Security and Cooperation in Europe (OSCE) said the law, introduced in 2007, has been expanded to bar over 5,000 sites in the past two years and is severely damaging freedom of expression and information rights.
I ask the Turkish authorities to revoke the blocking provisions that prevent citizens from being part of today's global information society, the OSCE's media freedoms chief Dunja Mijatovic said in a statement.
Turkey initially passed the law to restrict access to pornography and other content it deemed harmful to children. The Vienna-based, 56-nation OSCE says the law has now been used to go far beyond that.
Instead of allowing free access to the internet, new ways have emerged that can further restrict the free flow of information in the country, Mijatovic said.
Turkey, an OSCE member, first started blocking YouTube in 2008 after it ruled that some videos posted on the site were insulting to Mustafa Kemal Ataturk, founder of the modern republic.
The Turkish government has also cited offences including child pornography and encouragement of suicide for blocking websites.
The OSCE said Mijatovic had written to Turkey's foreign minister to complain about new restrictions introduced earlier this month that have hampered access to other Google services such as its instant translation site and web traffic tracker.
Mijatovic said the alleged reason behind the block was an unsettled tax row between Turkish authorities and Google but that this matter was not covered in the original law.
Earlier this month, Turkish President Abdullah Gul used his Twitter page to condemn the ban on YouTube and some Google services. He said he had asked responsible institutions for a solution. I asked for a change in regulations on merit.
The president's role in Turkey is largely ceremonial; decisions are taken by the prime minister and cabinet.
(Reporting by Sylvia Westall; Editing by Mark Heinrich)