Mainland police crossed the border and dragged activists and journalists to their side of the crossing at the Lo Wu immigration control point on Sunday, the detainees claim. Hong Kong police said they did not see the incident. Four activists and two journalists from Hong Kong were detained by mainland police for three hours after a protest at the immigration control point in support of jailed dissident Liu Xiaobo . Photographs and videos allegedly show plain-clothes mainland officers crossing the border at Lo Wu Bridge and grabbing activists' banners. Tsang Chun-ying, one of the four activists detained on Sunday, said men he believed to be mainland police approached them on the Hong Kong side of the border. When the first protester in a chain was pulled across the border, Tsang was also pulled over to the Shenzhen side. 'They did not tell us who they were, even after they took us to a room. We questioned their identities but they kept refusing to tell us,' he said. 'It was obvious they were plain-clothes officers as they worked with mainland police at the scene.' Ming Pao said photographer Yip Hon-wa was pushed across the border by plain-clothes officers and detained with colleague Leung Wai-kin, even after revealing their identities. Hong Kong police Chief Superintendent David Ng Ka-sing said he did not see any mainland officer exercising jurisdiction in Hong Kong, after reviewing video and photos from the scene. He said the situation was chaotic and several protesters crossed into the mainland side. Law Yuk-kai, of Hong Kong Human Rights Monitor, said the city's jurisdiction was clearly undermined. Legislative Council security panel vice-chairman James To Kun-sun said Secretary for Security Ambrose Lee Siu-kwong should give a public explanation. 'Why did police at the scene not take any action on the man in plain clothes who was taking away protesters' banners if he was not a mainland officer?' To asked. Audrey Eu Yuet-mee, of the Civic Party, said the case violated the 'one country, two systems' principle.