Three Hong Kong democracy protest leaders were Saturday denied permission to board a flight to Beijing, where they had hoped to bring their demands for free elections to Chinese authorities.

The leaders of the Hong Kong Federation of Students, spearheading mass rallies that have paralysed parts of the city for six weeks, said airline officials informed them that their permits used for travel to the mainland had been cancelled by Chinese authorities.

"Airline officials informed (the leaders) they did not have the required travel documents to get on the plane," Lester Shum, deputy secretary general of HKFS, told reporters.

Fruitless talks with the Hong Kong government almost a month ago have led to an impasse and protest leaders had planned to travel to Beijing to bypass the unpopular local administration altogether.

"We have received information from relevant departments on the mainland that the Home Return Permits of the passengers in question have been cancelled," a Cathay Pacific staff member told the trio, footage from Cable Television News showed.

The permit, issued by mainland authorities, allows Hong Kong residents free travel within mainland China, but some of the city's pro-democracy figures have been denied access to the country in the past.

"Residents holding valid travel documents shall be free to leave the Region without special authorisation," a spokeswoman for the city's immigration department told AFP on Friday.

Before they were turned back, the three leaders -- Nathan Law, Eason Chung and Alex Chow -- were mobbed by supporters who unfurled yellow umbrellas, a symbol of the city's democracy movement.

They also carried banners with pro-democracy messages including "we want real elections".

"Dialogue is important for resolving the current (situation) but it depends on whether Beijing has the initiative to open talks with students," HKFS leader Alex Chow said before he was turned back.

The former British colony was handed back to China in 1997 under the "one country, two systems" principle which promises to maintain the city's social and economic systems until 2047.

But democracy activists say Hong Kong's freedoms have been steadily eroded under Chinese rule.

The protesters are demanding civil nominations in leadership elections for the semi-autonomous city in 2017.

China has refused to back down on its insistence that candidates must be vetted by a loyalist committee, a decision critics say is designed to ensure the election of a pro-Beijing stooge.