They slammed their arrests as "ridiculous," questioning why they were being accused of breaching the national security law for merely standing in the primary.

But former lawmaker Wu Chi-wai was detained for a second night for breaching a court order to surrender all travel documents after police officers found a British National (Overseas) passport at his home.

The police's national security department arrested 53 people on Wednesday for allegedly plotting to overthrow the government by organizing or participating in a primary election last July.

They wanted to win at least 35 seats in the legislature and paralyze government operations as part of a "10-step mutual destruction plan" made by Occupy co-founder Benny Tai Yiu-ting and published last April, police said.

By late last night, those released included former lawmakers Helena Wong Pik-wan, Kwok Ka-ki and Jeremy Tam Man-ho, district councillors Lester Shum, Fergus Leung Fong-wai, Kalvin Ho Kai-ming, Lawrence Lau Wai-chung, Jimmy Sham Tsz-kit, and Lee Yue-shun, and activist Nathan Lau and Carol Ng Man-yee.

Others were expected to be released throughout the night.

After his release from Yau Ma Tei police station, Sham said: "I think it's ridiculous, as they didn't accuse me of subversion over what I did or said while running in the Legco election. They made accusations against me for merely standing in the primary."

Wu, 58, was brought to West Kowloon Magistrates' Courts yesterday afternoon.

The prosecution told principal magistrate Peter Law Tak-chuen that Wu, the former Democratic Party chairman, had breached his bail conditions in an earlier unauthorized assembly case by keeping his valid BN(O) passport.

He also allegedly incited people to take part in the July 1 rally last year.

Law adjourned the hearing to today and Wu was remanded in custody last night.

Yesterday, police made two more arrests in the case.

One is Joshua Wong Chi-fung, who is serving a 13-and-a-half month term in Shek Pik Prison for the siege of the police headquarters in June 2019. He was taken to Lai Chi Kok Reception Centre in handcuffs and waist chains to make a statement without a lawyer.

The vice chairman of localist group People Power, "Fast Beat" Tam Tak-chi, was also arrested by police inside Lai Chi Kok Reception Centre.

He has been remanded at the center since last September under sedition charges. His trial will begin in May.