Hong Kong's national security-themed museum exhibition has welcomed one million visitors within a year of opening, with an official urging people to always remember the pain stemming from the 2019 protests.

Sonny Au, secretary general of the committee for safeguarding national security, on Saturday said people should not "forget the pain once the wounds are healed".

Describing the one-million visitor mark as an "important milestone", Au said the city's national security exhibition gallery in Tsim Sha Tsui has been popular since opening its doors to the public in August last year.

The latest visitors were a group of primary school pupils, who called the experience "a rare opportunity".

Cheung Kin-kwong, who's in charge of the gallery, hoped to expand the exhibition's area and vowed to continue to look for "precious and symbolic exhibits" in the future.