HONG KONG (AFP) - Protesters turned Hong Kong's constitutional document into toilet paper on Wednesday in an angry demonstration against Beijing's publication of a controversial White Paper reasserting China's control over the city.

China's State Council - Beijing's Cabinet - on Tuesday issued its first-ever policy document stipulating how Hong Kong should be governed, in what was widely interpreted as a warning to the city not to overstep the boundaries of its autonomy.

It comes at a time of increasing political tension in the city as pro-democracy groups fear Beijing will backtrack on promised reforms.

About 40 demonstrators rallied outside Beijing's representative office in the west of Hong Kong, burning a large-scale reprint of the White Paper and brandishing rolls of toilet paper printed with the city's Basic Law.

"It is an explicit interference in Hong Kong affairs," pro-democracy lawmaker Lee Cheuk Yan told reporters at a separate protest outside the government's Liaison Office, as he urged Beijing to withdraw the document.

Hong Kong's unique status among Chinese cities is guaranteed by the One Country, Two Systems policy that allows it democratic freedoms and civil rights not permitted on the mainland.

The Basic Law is the city's Constitution, drawn up based on the terms of an agreement between Britain and China before the handover in 1997.

As part of democratic reforms promised by Beijing, the city will elect its leader for the first time in 2017.

But pro-democracy groups fear China's government will only allow its allies to be elected.

The White Paper warned that future leaders should not be "unpatriotic".

"The high degree of autonomy of Hong Kong Special Administrative Region is subject to the level of the central leadership's authorisation," it said.

It charged that some in the city were "confused or lopsided in their understanding of 'One Country, Two Systems' and the Basic Law".

"Many wrong views that are currently rife in Hong Kong concerning its economy, society and development of its political structure are attributable to this," it said.

The South China Morning Post said in a report headlined "A reminder of who's the real boss" that the White Paper showed Beijing's "determination to maintain control".

The Chinese-language Ming Pao in an editorial pointed to a "worrying situation" over growing Chinese influence, calling the document a "critical turning point" in Hong Kong's governance.

The White Paper came a week after tens of thousands of demonstrators gathered in a mass vigil to remember those killed in the Tiananmen Square crackdown on democracy protests in Beijing in 1989.

"It was an unprecedented move by the State Council aimed at deterring people from coming out to support real democracy," Civic Party leader Alan Leong told AFP on Wednesday.

Beijing was also seeking to quash the Occupy Central protests for universal suffrage with rallies planned in the core business district of Central, Mr Leong said.

A lobby group formed by barristers issued a statement refuting the White Paper's assertion that judges should safeguard national security and sovereignty.

The Hong Kong Bar Association said in the statement that judges should safeguard judicial independence as they are not the government's "administrators".