Thomson ReutersHONG KONG (Reuters) - Thousands of Hong Kong residents marched in protest on the 19th anniversary of the financial hub's return to Chinese rule on Friday as tensions simmer against Chinese authorities over the abductions of several Hong Kong booksellers. Some waved banners criticizing Beijing for the cross-border abductions as acts of a "totalitarian" regime, as well as calling for the release of leading dissidents. "This is a very grave threat to the safety of Hong Kong residents that an unknown force is spying on people," said pro-democracy lawmaker Cyd Ho at the rally. "The Hong Kong government has to follow up with the central government on what's really happening behind the scenes." The city has been unnerved over the past year by the disappearances of five booksellers who specialized in works critical of Chinese leaders.