President Imomali Rakhmon, a former head of a Soviet state farm, has ruled this Muslim Central Asian nation of 7.5 million people with a firm hand since 1992.
Although his government has shown a more liberal attitude to the press compared to repressive regional neighbors Uzbekistan and Turkmenistan, criticism of Rakhmon is taboo for the local media. Local access to the websites was blocked on March 2.
Dunja Mijatovic, the OSCE representative on freedom of the media, said in an appeal to the Tajik government she hoped that the ban on Facebook and the other sites would not set a precedent.
"Internet should remain an open public forum for discussion and free expression of opinions, as enshrined in the Universal Declaration of Human Rights," said the statement posted late on Wednesday on the site of the world's largest security body (www.osce.org).
She said she had sent a letter to Tajik Foreign Minister Hamrokhon Zarifi on March 5, and added: "I also expressed hope that access to Facebook and the four news websites would be restored without delay."
The Foreign Ministry declined to comment on the OSCE statement. March 8 is a market and public holiday in Tajikistan.
Facebook's popularity has soared in Tajikistan, with membership doubling last year to 26,000 people. Several Facebook groups openly discuss politics and some users have been critical of the authorities.
Tighter Internet controls in Tajikistan, the poorest of the 15 ex-Soviet states, echo measures taken by other ex-Soviet Central Asian nations, where authoritarian rulers are wary of the role social media played in the Arab Spring revolutions, as well as in recent mass protests in Russia.
The 59-year-old Rakhmon has overseen a referendum and a number of constitutional amendments that have extended his presidency. His current seven-year term will end next year, and he can seek another seven years in office to rule until 2020.
Authorities have launched a crackdown on religious groups in the impoverished mountainous state that borders Afghanistan and China. More than 150 people have been imprisoned in the last two years on charges of extremism and attempting to subvert the constitution.
Some of Rakhmon's critics argue that it is actually abject poverty and repression that push many young Tajiks to embrace radical Islam. More than 1 million Tajik citizens work in former imperial master Russia, sending home cash that helps the largely agrarian economy keep afloat.
Tens of thousands were killed in a 1992-97 civil war when Rakhmon's secular government clashed with Islamist guerillas.
(Reporting by Roman Kozhevnikov; Writing by Dmitry Solovyov; Editing by Alessandra Rizzo)