Wong, who was just 17 years old when he became the face of the 2014 student-led Umbrella Movement democracy protests, faces a maximum five-year jail term if convicted.
Before entering the courtroom, Wong said he would not be surprised if immediate detention followed.
"Perhaps the authorities wish me to stay in prison one term after another," Wong said.
"But I am persuaded that, neither prison bars, nor election bans, nor any other arbitrary powers would stop us from activism. What we are doing now is to explain the value of freedom to the world."
Wong was not a leading figure in last year's pro-democracy and anti-China protests, but his continued activism has drawn the wrath of Beijing, which sees it as a "black hand" of foreign forces.
He disbanded his pro-democracy group Demosisto in June, just hours after China's parliament passed a sweeping national security law for Hong Kong, punishing anything Beijing considers as subversion, secession, terrorism and collusion with foreign forces, with up to life in prison.
His long-time activist colleague Agnes Chow has already pleaded guilty to charges related to the same June 2019 protest, while Ivan Lam, another former Demosisto colleague was also expected to plead guilty.
Wong also faces charges of participating in an unauthorised assembly in October 2019 and on June 4, 2020 over a vigil commemorating the crackdown on protesters in and around Beijing's Tiananmen Square in 1989.
Earlier this year, Wong was disqualified along 11 other pro-democracy politicians and activists from running in a since-postponed election for the city's legislature.
Wong spent five weeks in jail last year for contempt of court, before being released on June 16 when protests were already in full swing.
Wong's and other activists' repeated arrests have drawn criticism from Western governments who say China is not fulfilling its obligation to allow Hong Kong a high degree of autonomy, agreed with former colonial master Britain when the city returned to Chinese rule in 1997.
China denies the accusation and says Hong Kong is its internal affair. - Reuters