The 12 who appeared at the District Court are among 20 people facing charges including taking part in an unauthorised assembly and inciting others to take part in the banned rally.
The other eight defendants did not appear in court on Friday because they had already told the court how they intended to plead, while another four activists – Joshua Wong, Tiffany Yuen, Lester Shum and Jannelle Leung – were in May jailed for between four and 10 months after pleading guilty early on.
On Friday, prosecutors said media tycoon Jimmy Lai and former Leung Yiu-chung would be pleading not guilty. Lai is already serving sentences for other protest-related offences and is also awaiting trial for alleged national security offences.
Prosecutors said those now intending to admit their guilt are veteran democrats Ho and Cheung Man-kwong, activists Figo Chan and Kwok Wing-kin, former lawmakers Wan, Chu, and Leung Kwok-hung, former district councillors Chiu Yan-loy, Leung Kwok-wah and Mak Hoi-wah.
In total, eight out of the 20 remaining defendants in the case have indicated they will deny the charges against them. Their trial is set to begin on November 1 and is expected to last 10 days.
The other 12 defendants are expected to officially enter their guilty pleas and submit mitigation during a court appearance on September 9.
More than half of the 20 defendants are already behind bars, having either been remanded in custody or jailed over separate protest-related offences.
As some of the accused were being escorted into the courtroom on Friday, friends, relatives and members of the public waved and shouted greetings at them.
Former lawmakers Emily Lau, Lee Wing-tat and Fernando Cheung, and activist Raphael Wong were among them.
“Take care Mr Lai, hang in there!” one shouted. Lai, in his light grey suit, and wearing a green mask, smiled and waved back.
“Dick, I miss you so much!” an old woman shouted, addressing Chu Hoi-dick in the dock.