Police arrested a man on Thursday who allegedly grabbed a security guard round the neck during a protest that disrupted a government forum on by-elections in September. The suspect was believed to have claimed online that night that the venue - the Science Museum in Tsim Sha Tsui - had been 'invaded', according to a source related to the case. He was wearing a Guy Fawkes mask when he joined dozens of others to storm the public consultation forum to discuss a proposal to scrap by-elections, the source said. The police arrested the 29-year-old man, identified only by his surname Yip, for common assault. It was understood that Yip was the person photographed choking the guard near the door, after the protesters - including League of Social Democrats lawmaker 'Long Hair' Leung Kwok-hung - broke into the venue. Soon afterwards, many internet postings emerged using the same name, apparently in a concerted effort to hide the true identity of the offender. But police intelligence led them to the suspect. He is on bail and must report to the police in the middle of next month. He has not been charged. Last month, five other protesters, including Leung, were sentenced to jail for disrupting the same forum. The lawmaker received a two-month jail sentence, while the others, including two university students, were given three-week prison terms. All have filed appeals against their sentences. Senior counsel and lawmaker Alan Leong Kah-kit said the Yip case was more difficult to prosecute, requiring sufficient evidence to prove the suspect was the person in a Guy Fawkes mask who assaulted the security guard. 'The cameras don't show the face behind the mask,' Leong said. 'I believe the police must have collected other evidence before making the arrest.' On Thursday, police said they had completed investigations into another instance of common assault at the same event and were awaiting legal advice from the Department of Justice.