Police fired choking volleys of pepper spray against protesters denouncing President Hu Jintao Saturday as he visited unruly Hong Kong to mark the 15th anniversary of its return to Chinese rule.

The incident underscored tensions surrounding the anniversary of the financial hub's handover from British control on Sunday, which will also see the inauguration of a new and unpopular leader of the local government.

On the second day of Hu's three-day visit, hundreds of protesters demanding an investigation into the recent death of a well-known mainland dissident rallied near the Chinese leader's five-star hotel.

Some tried to break through giant barricades, which are being used by Hong Kong police for the first time since they battled violent protests during a 2005 meeting of the World Trade Organization in the city.

Police unleashed riot-control measures to keep the demonstrators back, hitting a number of media personnel including three AFP correspondents in the process with eye-stinging pepper spray. They arrested two protesters.

As the standoff developed, other protesters chanted anti-Beijing slogans and unfurled a huge banner with the Chinese character "injustice" written on it.

Another Hong Kong reporter was briefly detained after shouting questions to Hu about the 1989 Tiananmen Square bloodshed, as the president visited the construction site for a new cruise-ship terminal earlier on Saturday.

Hu's visit comes as discontent toward Beijing surges to a new post-handover high, and his visit has drawn sneers and ridicule from Hong Kongers, who cherish freedoms in the city such as the right to protest not seen in China.

There has been palpable anger over the death of veteran Tiananmen activist Li Wangyang, who was found dead in his hospital ward in China earlier this month. His family say he died under suspicious circumstances.

The towering barricades in Hong Kong are likely to defy Hu's stated wish to "walk more" and "see more" during his trip to the semi-autonomous city, which returned to Chinese rule on July 1, 1997.

"Are we celebrating the handover anniversary or staging a war?" one Facebook user wrote.

The director of Hong Kong Human Rights Monitor, Law Yuk-kai, criticised the security measures as an attempt to shield the Chinese leader from any "embarrassing demonstrations". He said the city was "under siege".

Waving concert glow sticks, about 300 members of a pro-democracy party held a late-night vigil to mourn what they called an erosion of Hong Kong's freedoms and chanted slogans demanding universal suffrage.

The city maintains a semi-autonomous status with its own legal and financial system under the "one country, two systems" model that applies to both Hong Kong and the casino haven of Macau, across the Pearl River Delta.

Hong Kong police had vowed to respect the right to protest during Hu's visit, after they were criticised for heavy-handed tactics during a visit by Chinese Vice Premier Li Keqiang last year, prompting a special inquiry.

But some users on social media likened the security tactics laid on for President Hu to the Great Wall of China or the Berlin Wall.

"The Berlin Wall separated East and West Germany, let's hope this wall of barricades can separate Hong Kong from China forever," one anti-Beijing user wrote.

"Get out of Hong Kong!" another writer told the Communist Party and the People's Liberation Army (PLA) in a post on the Facebook page of the Hong Kong chief executive, which was flooded with anti-Beijing comments.

The remarks came a day after Hu attended a military ceremony that shed a rare light on the secretive PLA garrison stationed in Hong Kong.

On Sunday he will preside over the inauguration of new Chief Executive Leung Chun-ying, who has been dogged by alleged conflicts over his business interests and unauthorised renovations at his luxury home.

The cloud over Leung is emblematic of a widening income gap, and of the mistrust felt among Hong Kong's seven million people against the pro-Beijing business elites who, as part of a special committee, voted him into office.