Hong Kong pro-democracy activists Joshua Wong and Agnes Chow Ting were arrested on Friday ahead of planned protests, according to a post from the movement Demosistō.
Both were later released on bail but their detention was slammed as "an outrageous assault on the rights to freedom of expression and peaceful assembly".
"We will continue our fight no matter how they arrest and prosecute us," said Wong, secretary-general of Demosistō and a well-known activist in Hong Kong.
Andy Chan, another pro-democracy leader, was also arrested, according to the Hong Kong Free Press, citing police. Chan was arrested on suspicion of rioting and assaulting a police officer, the local media report said.
Chan's pro-independence political party was banned by authorities in 2018.
The arrests came ahead of planned protests this weekend.
According to his Demosistō, Wong was arrested on Friday morning while walking to a metro station. The group said he was pushed into a private minivan on the street.
Agnes Chow Ting, a university student and member of the group, was also arrested and sent to the Wan Chai police headquarters where Wong was also being held, the group said.
"This past week, we have seen scare tactics straight out of Beijing’s playbook: pro-democracy protest organisers attacked by thugs, prominent activists arrested after being snatched from their homes and streets, and a major rally planned for Saturday banned," said Man-kei Tam, the director of Amnesty International Hong Kong.
Tam called the arrests "an outrageous assault on the rights to freedom of expression and peaceful assembly".
Issac Cheng, the vice-chairperson of movement Demosistō, said Wong was arrested for inciting, organising and taking part in an unlawful assembly. Chow was arrested for inciting unlawful assembly.
Protests planned for Saturday, August 31, the five-year anniversary of a decision about elections that sparked the Umbrella Movement, were banned by police, citing concerns after violence broke out at protests, according to local media reports.
"Our planned demonstration for tomorrow, the fifth anniversary of 8.31. decision that blocked our way to democracy, has been objected by the police, and the appeal unanimously ruled unsuccessful today by the appeal board. Therefore, we are unable to call and will not be calling for a demonstration tomorrow," the Civil Human Rights Front, another Hong Kong organising group, wrote on Facebook.
Umbrella Movement protests in 2014 caused chaos in the city.
Protests this year in Hong Kong began in opposition to a proposed extradition bill that would have allowed prisoners to be sent to mainland China.
They escalated over the summer even after the extradition bill was scrapped by Hong Kong's government.
Now protesters are asking for an inquiry into police violence and are calling for democracy. They continue to demand that chief executive Carrie Lam step down.
Recent protests in Hong Kong's international airport resulted in chaos, violence and flight cancellations.