International investors plowed money into mainland China's main stock market on Monday after the debut of a cross-border trading link giving outsiders wider access through brokers in Hong Kong. In contrast, the flow of money in the opposite direction was just a trickle on the first day of trading on the Shanghai-Hong Kong Stock Connect. Officials in both cities banged gongs to mark the start of trading on the stock link, which lets investors buy and sell shares through each other's exchanges. "Today we are going to witness history," C.K. Chow, the chairman of stock exchange operator Hong Kong Exchanges and Clearing, said at an opening ceremony. "It is a breakthrough in the opening up of China's financial markets and an important milestone in the development of Hong Kong as a unique gateway between the mainland and international investors." The stock connect will give all sorts of investors outside mainland China access to the stock market in the world's No. 2 economy for the first time. Until now, access has been closely managed, mainly through a quota program for select fund managers representing a fraction of the overall market.By early afternoon, investors buying mainland Chinese stocks through Hong Kong in so-called "northbound" trading had used up nearly all of their daily 13 billion yuan ($2.1 billion) quota.The trading link also gives wealthy Chinese investors access to a market outside of the mainland for the first time. But Chinese investors had only used up just over a tenth of their daily 10.5 billion yuan ($1.7 billion) limit for Hong Kong stocks, according to data posted on the Shanghai Stock Exchange website.Trading is subject to daily and overall limits so that the stock exchanges can regulate the pace of turnover.Hong Kong Exchanges CEO Charles Lisaid the link was "a massive bridge, this is a massive road, and it is going to be here not for days, not for weeks, not even for months, it is going to be here for years and decades. At this point, safety, smooth travel is much more important than how many cars that actually cross the bridge."