Media playback is unsupported on your device Media caption The BBC's Juliana Liu and legislator Albert Chan Wai-yip explain why the protests are taking place

Tens of thousands of pro-democracy demonstrators have marched through the streets of Hong Kong as the former British colony marked 15 years since the return to Chinese rule.

The rally for human rights takes place annually, but has been bolstered this year by anger towards Beijing.

Earlier, China's visiting President Hu Jintao swore in businessman CY Leung as the territory's new leader.

During the ceremony, a lone heckler tried to interrupt Mr Hu's speech.

On the streets outside, massive crowds beat drums and waved flags as they marched though the city to call for full democracy and express their frustration with the mainland.

'Rule of law'

At the scene There was a carnival atmosphere, with pro-democracy political parties chanting slogans. Members of civic groups showed off their singing and dancing skills. And supporters of the Falun Gong spiritual group, which is banned in mainland China, sat peacefully in the lotus position, before joining in the protest with their marching band. Elaine Mok, a demonstrator who took part with her extended family, told me she marches nearly every year in order to fight for justice and the rule of law, and to oppose mainland interference in Hong Kong affairs. They were there, she said, to remind their Chinese overlords that Hong Kong people want the right to vote, as promised when this city returned to mainland rule. Most of the protesters were professionals like Ms Mok. Some families brought their young children. A broad cross-section of Hong Kong society gathered to agitate against one-party rule in China and to demand the right to universal suffrage, which people here increasingly believe is their natural birthright.

The BBC's Juliana Liu, who was at the protest, says there was a carnival atmosphere with political parties shouting slogans and civic groups showing off their singing and dancing skills.

One of the main complaints was that the system used to choose Hong Kong's leader is designed to install Beijing's choice.

A so-called electoral college of 1,200 business leaders and other influential citizens, mostly loyal to Beijing, selects the leader.

Elaine Mok, who was taking part in the protest with her family, said the march was about the right to universal suffrage.

"We're fighting for justice. We're fighting for the rule of law," she told the BBC. "The Chinese government is interfering with the workings of the Hong Kong government, and that's not right."

"We are fighting for the right to vote. It should have happened by now."

According to Paul Yip, a demographic specialist at the University of Hong Kong, some 82,000 people attended the rally - about 20,000 more than last year's demonstration.

Organisers, meanwhile, put the figure much higher, at 400,000.

'Joyous occasion'

Our correspondent says Mr Hu's visit was a far cry from his last appearance five years ago, when he toured Hong Kong in a blaze of pre-Olympic glory.

At the swearing-in ceremony, Mr Hu offered "warm congratulations" to the 57-year-old Mr Leung and his team and described the 15th anniversary as a "joyous occasion".

Media playback is unsupported on your device Media caption Hu Jintao offered greetings to the people of Hong Kong

He reiterated Beijing's commitment to the "one country, two systems" policy whereby Hong Kongers are allowed many more political freedoms than Chinese people on the mainland.

Mr Hu continued the address despite an interruption by a member of the crowd, who was heard calling for a condemnation of the 1989 Tiananmen Square massacre and an end to one-party rule in China.

The man, who was a guest at the inauguration ceremony, was quickly bundled out of the harbourfront building by security.

Mr Hu, whose visit was carefully choreographed, left before Sunday's protests began.

But on Saturday, police had to shield the president from demonstrators, and officers used pepper spray to disperse crowds who were demanding an investigation into the death in China of a Tiananmen activist, Li Wangyang, last month.

His visit comes as public confidence in the Beijing government has fallen to a new low.

People are unhappy with record property prices, an increasing wealth gap, a lack of democracy and a string of political scandals, our correspondent says.

Hong Kong, a British colony until 1997, has a comparatively high degree of autonomy from Beijing.

But China's leaders in Beijing have resisted public pressure for full democracy in the city.

Mr Leung replaces Donald Tsang, who took office in 2005.