POLICE using helicopters and sniffer dogs conducted a massive search of Kowloon Park last night, but failed to find two armed robbers who held up a Nathan Road jewellery shop. The two men wearing masks robbed the Tsim Sha Tsui store of almost $300,000 worth of gold jewellery. Helicopters from the Royal Hongkong Auxiliary Air Force, police dogs and more than 120 armed policemen were called in for a four-hour search of the area. Police sealed off Kowloon Park after witnesses reported hearing ''two shots'' fired from within the park. More than 1,000 people were evacuated from the park as police and their dogs swept the grounds. But no evidence that shots were fired was found. The two men held up the Sing Kwong Jewellery and Gold Company in Nathan Road, threatening the seven staff with guns. According to manager Mr Law Po-kong, the men were wearing masks and clad ''in jeans and casual wear''. ''I think they were mainlanders because they spoke an unclear Cantonese dialect,'' he said. No shots were fired during the holdup and no one was injured. When police arrived on the scene, they chased the robbers but called for more reinforcements when they lost the two men.