HONG KONG, June 30 (Reuters) - Hong Kong police re-arrested pro-democracy activist Chow Hang-tung after revoking her bail on Wednesday, on the eve of the anniversaries of the former British colony"s handover to Beijing and the centenary of the Chinese Communist Party. Chow, a barrister and vice-chairwoman of the group that organises annual vigils for the victims of China"s 1989 Tiananmen crackdown on pro-democracy protesters in Beijing, was arrested and released from custody earlier this month for promoting an unauthorised assembly on June 4. Police superintendent Chen Chi-cheong told a news conference that Chow had again been arrested for inciting others to participate in an unauthorised assembly. She was due to appear in court on Friday. Chow could not be reached for comment. Concerns are deepening over freedoms in the financial hub, which Britain returned to China on July 1, 1997, under a "one country, two systems" formula of government that guaranteed wide-ranging autonomy not seen in mainland China. Critics of the government say authorities have shrunk those freedoms and are now using a contentious national security law to crush dissent. Officials in Beijing and Hong Kong reject those assertions and say the law was necessary to plug gaping holes in the city"s national security defences exposed during anti-government protests in 2019. China marks the centenary of the founding of its ruling Communist Party on Thursday with pomp, spectacle, and what state media described as an important speech by President Xi Jinping in Tiananmen Square. Xi, China"s most powerful leader since Mao Zedong, and the party are riding high as the country recovers briskly from the COVID-19 outbreak and takes a more assertive stand on the global stage.