Three student leaders at the heart of the Occupy Central protests are demanding the Hong Kong government explain why their travel documents were invalidated yesterday, hours before they were to fly to Beijing to press their demands for true universal suffrage. It is extremely rare for authorities to revoke a Hongkonger's "home return permit" before they even reach the mainland. Local pro-democracy activists have in the past either had their entry denied without their travel permits affected, had their applications for renewal of the permits rejected, or had their permits confiscated on mainland soil. The students and pan-democratic lawmakers said the decision was an "affront to the law" and a sign that Beijing would not listen to Hong Kong people's views. This came as US President Barack Obama gave his strongest statement yet on the Occupy movement, saying that Hongkongers were demanding a universal, not Western, value. The students said they were "shocked and angry" to discover that their travel documents had been invalidated. "Why is a great state like China afraid of just three students?" federation secretary general Alex Chow Yong-kang said. "We have not even stepped on mainland soil." Watch: Hong Kong student leaders prevented from flying to Beijing to press for democracy It is the first time Federation of Students representatives have had their entry permits terminated; three times in the past 26 years, delegations have been allowed to visit over sensitive issues. Chow said he would demand an explanation from the Hong Kong government because it was only Cathay Pacific, the airline they were booked on, that told them their permits were revoked. The Security Bureau did not comment. An Immigration Department spokeswoman said the Airport Authority was responsible for the area where the three were turned back. The authority said it did not have the power to turn back the students. The students wanted to go to Beijing to seek meetings with state leaders including Premier Li Keqiang to express a demand for genuine universal suffrage for the 2017 chief executive election. They had three banners bearing slogans with Occupy protesters' signatures. At about 3.50pm yesterday, Chow and federation members Nathan Law Kwun-chung and Eason Chung Yiu-wa were escorted by airport staff through a crew entrance to have their baggage scanned and documents checked. They were told that they should use the crew entrance because the departure hall was packed with their supporters. But once inside, they were stopped by several police officers and Airport Authority staff and taken aside. After almost 30 minutes of questioning and waiting, Chow said they were told that they had to go back. A Cathay staff member told the students that the airline was "informed by relevant parties this morning" that their home return permits had been made void. Former federation member Jeffrey Tsang, who planned to take the same flight to help the trio with accommodation and transport, was also informed by Cathay that his travel permit had been invalidated. The decision was "an affront to the rule of law" and "arbitrary use of power", said Alan Leong Kah-kit, convenor of a weekly meeting of 23 pan-democratic lawmakers. He said he saw no point in the Hong Kong government starting the next round of public consultation on political reform as Beijing had stopped listening to the people. Watch: What do Hongkongers think about student leaders’ failed trip to Beijing and impending site clearance? But Regina Ip Lau Suk-yee, a lawmaker and former security minister, said mainland authorities had the right to revoke the travel documents of suspected terrorists, criminals or troublemakers. "The students are just staging a show. Their attitude is poor," Ip said. She said the trio's case could be compared to that of whistle-blower Edward Snowden, as the British government told airlines around the world not to allow him on board flights to the UK after the US government alleged he had leaked state secrets. Ip's comment echoed that of the Global Times , a state-run tabloid, which said in an editorial yesterday that the students' plan was "a show just to create an atmosphere of martyrdom". Meanwhile, Obama had words of support for the protesters who have occupied sites in Admiralty, Mong Kok and Causeway Bay for seven weeks. "Today, people in Hong Kong are speaking out for their universal rights," Obama said in a speech in Australia, where he was attending the G20 summit. "When we speak out on these issues, we're told that democracy is just a Western value; I fundamentally disagree with that." In response, the Chief Executive's Office reiterated that the government hoped foreign governments and legislatures would "respect" the principle that constitutional development was an "internal affair" of Hong Kong. Meanwhile, Markus Ederer, state secretary of Germany's Federal Foreign Office, told a law conference at the Chinese University of Hong Kong that people should "recognise and accept the legal and political limits", respect the rule of law and make compromises on political reform. He said he had met local officials and Beijing's representatives on Friday.