A liberal Chinese journal's website was shut down on Friday, sources said, in the latest and most prominent example of a crackdown by Chinese authorities against online freedom of expression.
The website of the Beijing-based Annals of the Yellow Emperor was closed days after it published an appeal for leaders to guarantee constitutional rights including freedom of speech and assembly.
The publication, which has links with senior retired Communist officials, had argued in the article that China's constitution lays out a road map for political reform.
Closure of the Annals' website follows censorship by the authorities of similar calls made by a key liberal newspaper, while several influential Chinese journalists have had their social networking accounts deleted in recent weeks.
The crackdown comes despite pledges of change from China's new Communist leadership, headed by president-in-waiting Xi Jinping, which has promised a more open style of governance since the ruling party's congress in November.
Attempts to access the Annals' website on Friday led to a page with a cartoon policeman holding up a badge and the message: "The website you are visiting has been closed because it has not been filed on record."
"At around 9 am today, the website was closed," said a post on the Annals' official web page on Sina Weibo, a website similar to Twitter, and later confirmed by the magazine's editors to the reprter. Editor-in-chief Wu Si said he received a message from the Ministry of Industry and Information Technology, China's Internet regulator, last month stating that the website had been "cancelled".
"I want to understand why the website was closed... we have been calling the relevant ministries all morning, and haven't had any answers," he said, adding that the magazine's print copies did not appear to have been seized.
The information technology ministry did not respond to faxed questions and phone calls made by the reporter on Friday.
The Annals magazine, which boasts former propaganda officials amongst its patrons, represents the views of the Communist party's more liberal faction, and its readership includes many within the party hierarchy.
The website closure came a day after censors blocked an article from popular liberal newspaper Southern Weekly which called for the realisation of a "dream of constitutionalism in China" so that citizens' rights could be protected.
A propaganda official in Guangdong province, where the newspaper is based, removed the piece and replaced it with a weaker message, said several current and former journalists at the newspaper.
The official, Tuo Zhen, "directed that many alterations and replacements be made to the New Year's special edition. This resulted in numerous errors and accidents", more than 30 former Southern Weekly journalists said in an open letter posted online.
In the letter they called for Tuo to resign, a demand echoed by another open letter from dozens of interns at the newspaper, in an unusually vocal and public response to censorship by the authorities.
David Bandurski, a Chinese media researcher at the University of Hong Kong, told sources: "The Southern Weekly incident is very important, very unprecedented... this kind of direct intervention by propaganda officials is something we haven't seen."
The clampdown was not unprecedented in China, where media has long been tightly controlled, but appears to contradict Xi's "new image", he added.
"I see this as the first test for Xi Jinping... it's basically saying: when you talk about openness do you really mean it?" he said.
Asked about the Southern Weekly article, a foreign ministry spokeswoman in Beijing said: "There is no so-called news censorship in China."
Several journalists known for criticism of the government found that their accounts on Sina Weibo were deleted last month, shortly before China -- which has the world's biggest population of Internet users -- implemented tightened online controls.
The new rules require Internet users to register with their real names, and service providers to remove posts with "illegal information" before forwarding them to authorities.
All Chinese media organisations are subject to instructions from government propaganda departments, which often suppress news seen as "negative" by the Communist Party, although some publications take a more critical stance.
The US-based China Digital Times website, citing a leaked directive, said the Central Propaganda Department ordered that "all reporters and editors... may not discuss" the censored Southern Weekly issue "on any public platforms".
China came 174th in a list of 179 countries ranked for press freedom in 2011-12 by the advocacy group Reporters Without Borders, falling three places compared to the previous year.