Hong Kong tycoon and pro-democracy advocate Jimmy Lai pleaded not guilty on Tuesday in his landmark trial, where he is accused of endangering China's national security, as prosecutors laid out details of what they said was collusion with foreign forces.

Lai, a leading critic of the Chinese Communist Party, faces two counts of conspiracy to collude with foreign forces — including calling for sanctions against Hong Kong and Chinese officials — under a China-imposed national security law.

"Not guilty," Lai said three times as each charge was read, appearing calm as he sat in a glass dock surrounded by guards and a court filled with supporters and foreign diplomats.