Taiwan's opposition Nationalist Party slammed the island's new government as "suppressive" for barring former president Ma Ying-jeou from travelling to Chinese-controlled Hong Kong on the grounds of national security.
Ma's China-friendly Nationalists lost landslide elections in January to President Tsai Ing-wen and her Democratic Progressive Party (DPP), which has traditionally leant towards independence from China.
It said barring Ma from visiting the former British colony of Hong Kong was "completely unhelpful for Taiwan's internal political reconciliation and social harmony".
"This suppressive style of the just-sworn-in DPP government is naked to all the people," it said.
Tsai's administration said on Sunday it had barred Ma from travelling to Hong Kong to give a speech at a media awards ceremony.
"Based on the international situation faced by Taiwan and in consideration of national security and interests, Hong Kong is a highly sensitive area (when it comes to) maintaining our national security," the Presidential Office said in a statement, adding that it would be difficult to control the risks of such a visit.
It added that the decision was made in after a review by a government committee that included foreign, defense, justice, China affairs and senior security officials and was in line with the island's national security regulations.
China has claimed sovereignty over Taiwan since 1949, when Mao Zedong's forces won the Chinese civil war and Chiang Kai-shek's Nationalists fled to the island. Communist Party rulers in Beijing have vowed to bring Taiwan under its rule, by force if necessary.
Hong Kong returned to Chinese rule in 1997 under a "one country, two systems" formula which guarantees wide-ranging autonomy for the financial hub but was the scene of lengthy and sometimes violent anti-China street protests in 2014 calling for fully democratic elections.
China had put forward the "one country, two systems" formula as a model for Taiwan to follow, which island leaders have rejected.
China has also repeatedly warned Taiwan of negative consequences if they fail to recognize Taiwan is a part of China under Beijing's "one China" principle.
Tsai has said she will maintain the status quo with China, but has not repeated the "one China" principle.