Appearing at the District Court, former lawmakers and activists admitted taking part in or inciting others to join an unlawful assembly in Victoria Park.
The 12 were former legislators Albert Ho, Andrew Wan, Cheung Man-kwong, Leung Kwok-Hung, Cyd Ho, Chu Hoi-dick and Yeung Sum, and activists Figo Chan, Steven Kwok, Chiu Yan-loi, Mak Hoi-wah and Leung Kwok-wah.
Eight other defendants in the case, including Apple Daily founder Jimmy Lai and Hong Kong Alliance vice-chairwoman Chow Hang-tung, had earlier told the court that they would not plead guilty. Their trial is set for November 1.
The court heard that some defendants had shouted protest slogans on the day in question, and pushed down barriers set up by police to allow people to enter Victoria Park.
Before pleading guilty, Chan shouted the lyrics of a protest song, “however strong the storm is, freedom still blossoms”.
In his mitigation plea, Albert Ho told the court that although they had broken the law, they were motivated by their conscience and moral commitment to keep up the tradition of mourning June 4, to remember the lessons of history and speak truth to power.
Ho said they had only wanted to exercise their basic freedom to mourn the victims of 1989.
He added that even if the Hong Kong Alliance disbands and the candlelight vigil is again banned in future, he believes Hong Kong people will continue to remember June 4 in their hearts.
Earlier, activist Joshua Wong and former district councillors Lester Shum, Tiffany Yuen and Jannelle Leung were sentenced to between four and 10 months in prison each after pleading guilty to taking part in an unauthorised assembly.
The court earlier also issued arrest warrants for two other defendants in the case, activists Nathan Law and Sunny Cheung, who left Hong Kong last year.