A top Chinese official said Hong Kong can have "peace of mind" that its autonomy will be preserved, but hit out at independence activists during a highly-charged visit that sparked protests Wednesday.

The three-day trip by Zhang Dejiang, who chairs China's communist-controlled legislature, is the first by such a senior official for four years. It comes as concerns grow that freedoms are under threat in semi-autonomous Hong Kong as China tightens its grip.

Zhang's visit was ostensibly for an economic conference Wednesday, but has been widely seen as a conciliatory effort as frustration over lack of political reform has sparked a fledgling independence movement, condemned by authorities in both Hong Kong and mainland China.

At a banquet for legislators Wednesday but boycotted by pro-democracy lawmakers, Zhang said Hong Kong would not be "mainlandised" by China.

"Those saying the central government wants to mainlandise Hong Kong or even turn 'one country, two systems' into 'one country, one system' have no grounds," he said, referring to the city's semi-autonomous system of government.

"'One country, two systems' is in the best interest of the country and Hong Kong. The central government will implement it unwaveringly. Hong Kong society can have a complete peace of mind."

However, Zhang criticised the city's "localism" movement which is calling for more autonomy from the mainland, with some campaigners calling for a complete breakaway.

"There is a minority of people advocating for the independence of Hong Kong and resisting the central government," said Zhang.

"It is not a question of localism, it is an effort to separate the city from China under the name of localism," he added, saying society should "strongly condemn" actions that breach the rule of law.

Before the banquet, Zhang had met lawmakers, including four pro-democracy legislators, after promising to listen to political demands from across society.

The lawmakers said they had expressed their views to Zhang, but said he gave only standard replies.

"The overall atmosphere was very civilised but (there was) nothing unexpected," said Civic Party legislator Alan Leong.

"Zhang seemed to brush aside our observations."

- Protest anger -

Roads around Zhang's hotel and the convention centre where he attended the conference and banquet were cordoned off with huge water-filled barricades, infuriating protesters who were kept out of sight in designated areas.

Around 200 pro-democracy protesters and rival pro-China demonstrators gathered in one of the zones Wednesday evening as Zhang gave his banquet speech.

Beijing supporters waved national flags and played patriotic songs on loudspeakers.

Pro-democracy supporters shouted at them to "go back to China" as well as calling for free elections and an end to one-party rule.

They expressed anger they could not make themselves heard to Zhang.

"Zhang didn't come here to hear the real people's views," veteran activist Lee Cheuk-yan told the crowd, asking him to come to the protest area.

Some also questioned why there was such a massive police presence -- thousands of officers were mobilised to protect Zhang.

On Tuesday police arrested seven members of the pro-democracy League of Social Democrats party for unfurling protest banners on hills and flyovers.

They also wrestled a leading pro-democracy activist to the ground near Zhang's hotel as he tried to breach a barrier.

Human Rights Watch said Wednesday authorities had "sharply limited" the public's opportunities to voice criticism of Zhang's visit.

It added Hong Kong officials should challenge Zhang "to make concrete commitments to respect Hong Kong's autonomy on human rights and democratic rule".

Hong Kong is semi-autonomous after being handed back to China by Britain in 1997 and enjoys freedoms unseen on the mainland, but there are concerns Beijing's interference is growing in a range of areas, from politics to education and the media.

Zhang's is expected to visit a public housing estate Thursday after saying he wanted to understand the lives of Hongkongers before leaving Thursday afternoon.