Thousands of students in Hong Kong have converged on a university campus to demand greater democracy as they started a week-long boycott of classes. 

They are protesting against Beijing's decision to reject fully democratic elections in the former British colony.

Students from more than 20 universities and colleges gathered for a sit-in at the Chinese University of Hong Kong, several kilometres north of the city centre.

Some were holding banners saying: "The boycott must happen. Disobey and grasp your destiny."

Most of them were wearing white T-shirts with yellow ribbons - a colour adopted by Hong Kong pro-democracy activists. 

"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 boycott who also called on Hong Kong to "reject fake elections".

Pre-screened candidates

Beijing decided last August to rule out open nominations for candidates under proposed guidelines for the
first elections, so far, to choose the city's top leader, which have been promised for 2017. 

The National People's Congress, China's legislature, instead insists that candidates be vetted by an elite committee that is similar to the body of mostly pro-Beijing elites that has, until now, selected the city's leaders.

The university boycott comes as some of Hong Kong's most powerful tycoons visit Beijing to discuss Hong Kong politics with Chinese leader Xi Jinping.

The boycott is part of a larger protest that will take place on October 1 planned by pro-democracy group Occupy Central.

A survey by the Chinese University found that more than a fifth of Hong Kong residents are considering leaving the city, spurred by concerns about its political future.

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.