A DISTRICT board member and a Legislative Councillor's aide who took part in a demonstration outside Xinhua (the New China News Agency) should not be found guilty of unlawful assembly, an offence which infringed the Bill of Rights, it was submitted in the Court of Appeal yesterday. Wong Tai Sin District Board member To Kwan-hang and assistant to Lau Chin-shek, Richard Tsoi Yiu-cheong, argued that the magistrate who convicted them erred by not striking down the law which created the offence of unlawful assembly. Through Martin Lee QC, their counsel on appeal, they contended they were justified to use force to break through a police cordon that was set up ''illegally'' on June 4 and 5, 1991. The two men had both denied a charge of unlawful assembly, an offence under Section 18 of the Public Order Ordinance. Under this provision, unlawful assembly is defined as when three or more people, assembled together, conduct themselves in a disorderly, intimidating, insulting or provocative manner intended or likely to cause any person reasonably to fear they will commit a breach of the peace. They were found guilty after trial by Magistrate Alan Wright and were each sentenced in September last year to 160 hours' community service. As no application for a stay of the order was made, Mr To has served his sentence while Mr Tsoi has completed two-thirds of his penalty. It was the Crown's case that on the evening of June 4, 1991, the Hong Kong Alliance in Support of the Patriotic Democratic Movement in China held a candlelight vigil in Victoria Park, a peaceful assembly attended by about 20,000 people. At the end of the meeting, some participants marched to the Xinhua building in Happy Valley where police had set up a cordon. It was alleged that items such as plastic bottles were thrown and there was ''a tremendous noise''. According to Chief Superintendent Lionel Lam Kin, then deputy regional commander for Hong Kong Island, warnings to disperse peacefully were issued but the pushing increased and Mr Tsoi and Mr To were arrested. Maintaining the stance taken by the defence at the trial, Mr Lee, on appeal, argued that the magistrate erred in law in holding that Article 17 of the Bill of Rights, which guaranteed the right of peaceful assembly, was consistent with Section 18 of the Public Order Ordinance. He said there was no evidence to show the demonstrators had put anyone in danger. A confidential standing order made about 20 days after the incident by the commander who took over from Mr Lam, said Xinhua had become the focal point for demonstrations. It said most demonstrations had been conducted with reasonable order, and although they at times became heated, they nevertheless stayed within the law. Mr Lee said that while the standing order was not binding on the trial magistrate, it was, however, the official view of the police and was contrary to Mr Lam's evidence. Far larger demonstrations had since been held outside Xinhua and no police cordons had been set up, Mr Lee said. The hearing continues today before Vice-President Mr Justice Macdougall, Mr Justice Litton and Mr Justice Bokhary. About 20 members of the Hong Kong Federation of Students marched to the Court of Appeal to support Mr To and Mr Tsoi. and demonstrate against the Public Order Ordinance.