Supporters of one of China's most liberal newspapers have demonstrated outside its headquarters in a rare protest against censorship.

The protest in Guangzhou, capital of southern Guangdong province, comes amid an escalating standoff between the government and the people over press freedom.

It is also an early test of Communist Party Chief Xi Jinping's commitment to reform.

The outcry began last week after reporters at the Southern Weekly newspaper accused censors of replacing an original New Year's letter to readers that called for a constitutional government with another piece lauding the party's achievements.

Police allowed the demonstration outside the headquarters of the Southern Group, illustrating that the Guangdong government wants to tread carefully to contain rising public anger over censorship.

The protesters laid down small hand-written signs that said "freedom of expression is not a crime" and "Chinese people want freedom".

Last night, the Southern Weekly official microblog denied the removal of the New Year letter was due to censorship, saying the "online rumours were false".

Those remarks drew criticism from Chinese internet users.

Many Southern Weekly journalists disavowed themselves from the statement on the microblog, which they say was taken over by management, and pledged to go on strike.

Several open letters have circulated on the internet calling for the Guangdong propaganda chief, Tuo Zhen, to step down, blaming him for muzzling the press.

Chinese internet users already cope with extensive censorship, especially over politically sensitive topics such as human rights and elite politics.

Popular foreign sites Facebook, Twitter and Google-owned YouTube are blocked.

China shut the website of a leading pro-reform magazine last week, apparently because it ran an article calling for political reform and constitutional government.

The attention paid to today's protest domestically highlights the unique position of Guangdong, China's wealthiest and most liberal province and the birthplace of the country's "reform and opening up" programme.

In a symbolic move, Mr Xi chose to go to Guangdong on his first trip after being appointed party chief in November.