Roundtable lawmaker Michael Tien has urged the authorities to provide a better explanation for their arrest of 53 pro-democracy figures on Wednesday, saying he can't see how the group could have violated the national security law just by holding primary elections.

The suspects, including former lawmakers, activists and alleged organisers of the pro-democracy camp's primaries in July, are all accused of subversion.

Tien, who is a deputy of the National People's Congress, said he understands the authorities' issue with the group hoping to win a majority of seats in Legco in order to vote down government budgets.

But he noted that when it comes to subverting state power, the national security law sets out how any offences would involve the use of force, threats of force, or other unlawful actions.

Tien said the authorities should explain what unlawful means they suspect were involved in this instance.

"On the surface, holding a primary doesn't seem to be unlawful, and if they did get into Legco, pressing a button and casting their vote is not unlawful... so the point in contention now is not the end, but rather the means," he said.

He questioned whether the group could be convicted if the authorities did actually decide to lay charges against them.

"Looking at the way the law is written, I don't see how these people can be convicted, unless the court interpreted the ultimate motive as part of the bill... If that's the case, then this massive arrest will result in another slap in the face for the government," he said.

He said that scenario would only make Hong Kong more divisive, as it would show that the government was prosecuting people without understanding the law.

The pro-Beijing lawmaker added that the national security law might also need to be redrafted.

"Is there something wrong with the drafting of the national security bill, if the original intention is to do with the motive, rather than the methodology?... If they are concerned about the motives and the ends of all these acts, why did they bother to put in the means? That would only complicate the matter," he said.