Tens of thousands of protestors filled the streets of Hong Kong Sunday, adamant there was nothing to cheer as the former British colony marked 15 years of Chinese rule and swore in a new leader.

The vast rally came after Leung Chun-ying, a millionaire property consultant seen as close to China's communist rulers, took the chief executive's oath in front of Chinese President Hu Jintao -- who had his speech interrupted.

Hu's visit and Leung's inauguration have become focal points for growing discontent towards Beijing, which has surged to a new post-handover high amid soaring housing costs, limited democracy and perceived meddling by China.

"Hong Kong has become much worse off," Eric Lai of the Civil Human Rights Front told the marchers. "Our rights are under serious threat."

The crowd ranged from engineers and civil servants to maids and students, and represented groups from the Falun Gong spiritual movement to trade unions, as well as ordinary citizens young and old.

In sweltering heat it took them nearly three hours to file out of the park where they gathered, organisers said, and they blocked streets far across the city, stranding buses and trams as they surrounded them.

Mostly clad in the mourning colours of black and white, they carried placards calling for "One person one vote" and chanted "Power to the people", sometimes in more of a carnival atmosphere, complete with drums and songs.

The financial centre enjoys significant autonomy and civil liberties unheard of on the mainland under the "one country, two systems" model covering its return to China in 1997 after more than a century of British rule.

But marcher Jacky Lim, 37, who carried Hong Kong's former colonial flag bearing the British union jack, said: "There is nothing worth celebrating today. Hong Kong is being gradually destroyed by the Communist Party.

"The direct interference of Beijing in the election of Leung Chun-ying is a clear example," he said.

Hong Kong does not yet choose its leader by universal suffrage, and Leung was elected as chief executive in March by a special committee stacked with pro-Beijing business elites.

A spokesman for the Hong Kong government said it "fully respected" freedom of expression and the right to "take part in processions", and would listen to the demonstrators' views "in a humble manner".

Organiser estimates of crowd size were not immediately available because it was so large, while police gave a preliminary figure of 55,000, although that was likely to rise.

Earlier, as President Hu began his speech to around 2,300 guests at Leung's inauguration, a protestor inside the harbourfront venue repeatedly shouted "End one-party rule".

The man also referred to the crushing of democracy protests on Tiananmen Square in Beijing on June 4, 1989, and was rapidly bundled away by security personnel, while the audience drowned him out with extended applause for Hu.

Beijing's support for "one country, two systems" and the right of the people of Hong Kong to rule the territory was "unwavering", said Hu.

"We will follow the Basic Law... to continue to advance democratic development in Hong Kong," said the president, who will step down as part of a once-in-a-decade leadership transition in Beijing starting later this year.

Hu -- who said Friday he hoped to "walk more" and understand Hong Kongers' "life and expectations" -- left for Beijing before the march began.

Stifling security was imposed for his three-day visit, with police using pepper spray on demonstrators at one point on Saturday, and briefly detaining a Hong Kong reporter who shouted a question about Tiananmen at Hu.

China's economic rise has helped spur impressive growth in Hong Kong and boost the city's status, and supporters packed a stadium Sunday for a gala featuring a People's Liberation Army parachute display.

But tensions are growing between the seven million locals and their northern neighbours, with newly rich Chinese mainlanders accused of everything from pushing up property prices to monopolising schools and maternity beds.

A poll released by Hong Kong University last week showed mistrust towards Beijing at 37 percent, a post-handover high, and the number of Hong Kongers identifying themselves primarily as citizens of China plunged to a 13-year low in another survey.

Discontent against the local authorities is also widespread amid complaints about a widening gap between rich and poor.

Leung has promised to tackle the grievances but ahead of his swearing-in, a group of demonstrators burned his portrait.

"If we work together, I am sure Hong Kong -- the Pearl of the Orient -- will sparkle again," Leung said in his