Hong Kong pro-democracy media tycoon Jimmy Lai was arrested Monday and led in handcuffs through his newspaper office as police raided the building, part of a sweeping crackdown on dissent since China imposed a security law on the city. Lai, 71, was among seven people detained on suspicion of colluding with foreign forces -- one of the new national security offences -- and fraud in an operation targeting the Next Media publishing group. It was the latest police operation against dissidents since Beijing imposed the law on Hong Kong at the end of June. Two of Lais sons were among those detained, a police source told AFP. Journalists working at Lais Apple Daily took to Facebook to broadcast dramatic footage of police officers conducting the raid, and the newspapers chief editor Law Wai-kwong demanding a warrant from officers. "Tell your colleagues to keep their hands off until our lawyers check the warrant," he was filmed saying. Apples staff were ordered to leave their seats and line up so police could check their identities as officers conducted searches across the newsroom. At one point Lai was present, in handcuffs and surrounded by officers. Police said the search was conducted with a court warrant which was shown to staff. Chris Yeung, president of the Hong Kong Journalists Association, described the police action as "shocking and terrifying". "This is unprecedented, and would be unimaginable only one or two months ago," he said. Apple editor Law later sent a note to staff, seen by AFP, telling reporters to "stand by their posts" as he vowed to get the latest edition printed despite the raid. The security law was introduced in a bid to quell last years huge and often violent pro-democracy protests, and authorities have since wielded their new powers to pursue the citys democracy camp, sparking criticism from western nations and sanctions from the United States.