While the police did not identify those arrested, and only confirmed they were all students, a member of Student Localism, the disbanded group, said its 19-year-old former president, Tony Chung, was among those arrested.
The federal government stated the relocation was made "in order to protect public safety and public health as well as ensure elections are conducted openly and fairly".
Avery Ng, secretary-general of the League of Social Democrats, was equally sceptical. "Maybe there won't be any voting ever again, and they will just recruit some of their lackeys to form a provisional LegCo, meaning that the government can do anything it wants".
The statement noted that more than 60 governments around the world - such examples have included Singapore, South Korea and Mongolia - had already successfully held elections during the pandemic.
Supporters say it will restore some stability to Hong Kong after a year of protests.
Opposition pro-democracy groups hold 22 of the seats and had hoped to improve on that count and even win a majority by capitalizing on opposition to the new legislation. Noting expectations of high voter turnout, she invoked emergency powers to push the election back, calling it "a very hard decision". China's parliament would decide how to fill the legislative vacuum, she added. "The EAC understands and respects the government's decision, and will coordinate on the practical arrangements for postponement", said a spokesman for the EAC. Lam said he had the support of the Chinese government in this decision.
Around half of Hong Kong's almost 3,300 COVID-19 cases have been detected in the past month alone and authorities fear hospitals are on the verge of being overwhelmed.
A postponement would be a setback for the pro-democracy opposition, which was hoping to capitalize on disenchantment with the current pro-Beijing majority to make gains. Over one-third of them were reported over the past two weeks.
But the opposition camp and activists believe the law not only spells the end of the city's high degree of autonomy but also paves the way for a crackdown on dissent.
"China has never interfered in the internal affairs of other countries, including the United Kingdom, and we ask the same from other countries", Liu said. "I'm disappointed but I was ready for it".
Political activist Ventus Lau Wing-hong (third from left) speaks to reporters after he was disqualified on Thursday from running for election for the Hong Kong legislature.
"Barring me from running. would not stop our cause for democracy", said Wong, 23.
Earlier Thursday, pro-democracy activist Lee Cheuk-yan criticised the new national security law imposed by China following protests in Hong Kong that went on for months.
It said candidates who advocate self-determination, support foreign intervention, oppose in principle the enactment of the national security in Hong Kong "could not genuinely" uphold the Basic Law, Hong Kong's de facto mini constitution.
Political scientist Kenneth Chan fears that more disqualifications are on the way.
More candidates are expected to be barred in coming days.
Critics decry the decision, seen as the latest in a series of recent moves that curb Hong Kong's limited autonomy.