The increased vigilance came a day after one of the windows at the court was cracked by pellets fired from an air gun.

Since the first day of the national security trial on Monday, a heavy police presence has been evident inside and outside the court, but the level of security went up a few notches yesterday with hundreds of officers - in uniform or otherwise - deployed along with dogs.

Roadblocks were also set up on a section of Tonkin Street adjacent to Sham Shui Po Park swimming pools.

On Wednesday, a five-meter tall glass panel - part of a big window stretching from the first to the third floors of the court building - was found cracked, with a three-centimeter hole likened to a bullet hole in the top left corner.

The trial was being heard in a courtroom on the fourth floor. Police believe the window was hit by pellet-like projectiles.

On the fourth day of the trial yesterday, deputy director of public prosecutions Anthony Chau Tin-hang continued his opening address and fleshed out the roles played by 11 out of 16 defendants who pleaded not guilty, after he spoke about seven of them on Wednesday.

The 16 are among 47 pan-democracy figures charged with conspiracy to subvert state power between July 2020 and January 2021. The 31 others have pleaded guilty.

Speaking before national security law judges Andrew Chan Hing-wai, Alex Lee Wan-tang and Johnny Chan Jong-herng, Chau said three of the defendants had initiated a "resistance declaration," which he described as a key document that pledges to veto the budget in the Legislative Council and force the government to address five demands by protesters.

Of the 16, only three - then lawmakers Helena Wong Pik-wan, Lam Cheuk-ting and "Long Hair" Leung Kwok-hung - had not signed it, Chau said.

However, in Lam's notebook which officers seized, he wrote: "[I] completely agree with the ideal of Liberate Hong Kong, Revolution of Our Times."

Chau said Lam had also told fellow opposition politicians he would sign the declaration if necessary and would veto the budget.

In Leung's case, Chau said he had written articles about his party League of Social Democrats' support for "complete opposition" in Legco.

He had also publicly advocated organizing large-scale civil disobedience and strikes to arouse international attention.

The trial continues today to discuss the legitimacy of the prosecution's request to summon a social media expert as a witness.

Prosecution witness, including four defendants who had pleaded guilty, will begin to testify in court from next Monday.

The judges yesterday warned the defendants and spectators not to disturb witnesses on pain of legal consequences.