Those found guilty of taking part in an unlawful assembly could face as long as five years in prison depending on the severity of the offense.
Wong rose to prominence as a student leader during the 2014 Umbrella Movement pro-democracy protests and is among a growing number of activists being charged for various relatively minor offences since Beijing in June imposed a sweeping national security law on the territory that has severely restricted political expression.
Pro-democracy supporters have said the legal charges are part of a campaign to harass and intimidate them. Lam, who also spoke ahead of the court hearing, said he too was prepared to be jailed.
Wong wrote on social media on Sunday that he and Lam had decided to plead guilty after consulting with their lawyers. The two previously pleaded not guilty to the charges.
Another activist, Agnes Chow, has already pleaded guilty to charges of inciting others and taking part in the protest.
"If I am sentenced to prison this time, it will be the first time in my life that I have been in jail," Chow wrote on her Facebook page on Sunday.
"Although I am mentally prepared, I still feel a little bit scared. However, compared to many friends, I have suffered very little. When I think of this, I will try my best to face it bravely," she wrote.