Thousands of students in Hong Kong have protested against Chinese government plans to vet candidates for the next leadership election.

Student groups, who braved the sweltering heat, said the authorities in Beijing are backing out of a commitment to greater democracy, given when Britain handed Hong Kong back to China in 1997.

The class boycott coincides with a trip to Beijing by some of Hong Kong's most powerful tycoons, who are are expected to discuss Hong Kong politics with Chinese leader Xi Jinping.

Wearing yellow ribbons, students from more than 20 universities and colleges packed into the grounds of the Chinese University of Hong Kong, where they were greeted by banners that said: "The boycott must happen. Disobey and grasp your destiny."

Some of the student leaders wore black T-shirts with the words "democracy now".

Some of the students were chanting "united we stand" and "democracy now". ( Reuters: Tyrone Siu )

The former British colony returned to Chinese rule with a high degree of autonomy.

But last month, Beijing rejected demands for people to freely choose the city's next leader, prompting threats from activists to shut down the central finance district as part of their pro-democracy campaign.

"We demand the government responds to our call to endorse civil nominations," said Alex Chow, leader of the Hong Kong Federation of Students, one of the organisers of the classroom boycott.

President of the Hong Kong University Students' Union, Yvonne Leung, said the protest was the start of a wider campaign.

"Perhaps in the near future students or other NGOs will initiate another huge wave of civil disobedience to call for genuine democracy," she said.

"So we are evaluating the actions in a series instead of only the strike alone. But the strike is acting as a transitional point."

The students converged on a long boulevard at the university, with some carrying umbrellas to protect them from the baking sun, and many chanting "united we stand" and "democracy now".

Some academics in Hong Kong have voiced support for the strike, with some offering to record lectures and post them online for students who miss school to watch later.

One in five residents would leave Hong Kong: survey

Meanwhile, more than one in five Hong Kong residents would consider consider emigrating as pessimism grows over the island's political independence, a new survey shows.

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A poll conducted by the Chinese University of Hong Kong showed over 21 per cent of respondents said they would consider leaving the city altogether given the current climate.

"Hong Kong is on the brink of another wave of mass migration," said Sonny Lo, a professor at the Hong Kong Institute of Education who is unaffiliated with the poll.

"More and more people will leave in the next few years as the election for the chief executive approaches."

The city saw waves of migration leading up to the transfer of sovereignty from the UK to China, when tens of thousands left every year.

Emigration also surged in the wake of the Tiananmen crackdown in 1989, when Beijing violently suppressed protests by thousands of students calling for democracy.

Managing Hong Kong is proving a challenge for Beijing, which is worried that calls for democracy in Hong Kong and the nearby former Portuguese colony of Macau could spread to cities on the mainland, threatening the Communist Party's grip on power.

AFP/Reuters