HONG KONG (Reuters) - Thirteen Hong Kong universities and academic institutions accused the Chinese-ruled city’s leader of undermining freedom of expression amid a row over pro-independence banners appearing on campuses.

A banner in Chinese which reads "Hong Kong Independence" is displayed at the Chinese University of Hong Kong in Hong Kong, China September 8, 2017. REUTERS/Bobby Yip

Hong Kong, a former British colony that returned to China in 1997, is guaranteed freedoms and a high degree of autonomy under a “one country, two systems” arrangement, including freedom of expression.

At the start of the academic year, banners advocating independence from China appeared on noticeboards in at least seven universities. Some large black banners were hung across buildings.

Hong Kong leader Carrie Lam criticized the posters as a violation of China’s sovereignty, while urging university administrators to take “appropriate action”.

Some colleges, including the prestigious Chinese University, described the posters as unconstitutional, but allowed some to remain.

But late on Sunday, the 13 institutions issued a statement titled “Arming ourselves in our darkest hour”, criticizing Lam and university authorities for “an explicit effort to limit our freedom of expression”.

“Student unions stress that everyone enjoys the freedom of speech, and this is the line that we shall never compromise ... we are ready to defend our rights and liberty,” it read.

Slideshow ( 3 images )

Some observers said the controversy could be used to justify another squeeze on the city’s freedoms, soon after several young pro-democracy leaders were jailed for helping lead the city’s massive “Occupy” pro-democracy civil disobedience movement in late 2014.

The row has also stoked tension between local and mainland students, who now comprise a sizeable part of university admissions, especially in post-graduate studies.

Calls for independence, once rare in the financial hub, began to gain traction after the 2014 protests and as disillusionment grew toward China’s perceived tightening grip. Late last year, two pro-independence lawmakers were disqualified from office after Beijing’s parliament ruled their oath-taking carried digs at China.

Beijing resolutely opposes talk of Hong Kong splitting from China, with the mini-constitution stating the city is an “inalienable” part of the country. The so-called Basic Law also enshrines freedom of expression.

Groups of students from both sides have faced off on several occasions, with mainland students putting up anti-independence posters, condemning calls for independence.

One female student from China was filmed and challenged for tearing down some of the pro-independence banners.

“If you’re talking about democracy, you can put them up (the banners) and I can pull (them) down,” she said in the video.

An official blog run by China’s state mouthpiece, the People’s Daily, on Sunday published a long editorial saying there were limits to freedom of expression and that Hong Kong laws on public order could be used to jail trouble makers.

“It is quite apparent that Beijing and the Hong Kong government would like to use this excuse to impose a political crackdown,” said political commentator Joseph Cheng.

“Certainly the pro-Beijing establishment has been asking for rapid legislation of the controversial Article 23 legislation,” Cheng added, referring to proposed national security laws that would criminalize perceived acts of sedition.