Hong Kong pro-democracy protesters have draped a black flag over a statue symbolising the city's return to China by Britain, just days before a visit by President Xi Jinping to mark 20 years since the handover.

Joshua Wong, a prominent student campaigner, and a dozen demonstrators attached the black cloth to the giant golden bauhinia flower on Hong Kong's harbourfront in an early morning protest on Monday as security guards tried to stop them climbing on the famous tourist attraction.

"The black-cloaked statue symbolises the hard-line rule of the authoritarian regime over the past twenty years," the protesters said in a statement.

The sculpture of the bauhinia, which became the emblem of Hong Kong after the handover, was a present to the city from China in 1997 and stands outside the convention centre where Xi will attend anniversary events during a three-day visit starting on Thursday.

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Police were called to take the flag down while the protesters chanted "democratic self-determination for Hong Kong's future" and "one country, two systems has been a lie for 20 years", referring to Hong Kong's semi-autonomous status.

A guard shouted at them: "You are insulting our country! You are Chinese!"

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The "one country, two systems" deal made when Britain handed Hong Kong back to China in 1997 allows the city rights denied on the mainland, including freedom of speech.

But there are increasing concerns that China is trampling on the agreement by interfering in a range of areas, from politics to education and media.

Campaigners like Wong are calling for democratic reforms, promised in the handover deal, to change a system where the city leader is still chosen by a pro-China committee and the legislature is weighted towards Beijing.

Wong led mass Umbrella Movement rallies calling for fully free leadership elections in 2014, but they failed to win concessions.

Since then calls for self-determination or even full independence from China have emerged for the first time.

Wong's party Demosisto wants a public referendum on Hong Kong's future in 2047, the year the handover agreement guaranteeing the city's way of life and liberties expires.

"The protest action aims to express our anger and disappointment against the administration for the major political blunders since 1997," Demosisto said in a statement.

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It accused China of failing to honour promises made in the handover agreement, "depriving Hong Kong people of civil and political rights to free elections and democracy".

Xi's visit will be his first since becoming president in 2013 and will culminate with the inauguration of Hong Kong's new leader, Carrie Lam, on Saturday.

Protesters say they are preparing to gather during the handover celebrations and Xi's visit will be shrouded in a huge security operation.

Source: News agencies