What began as a peaceful gathering lead to a small group of demonstrators setting off fire extinguishers and barriers.
Although the assault happened in June, the force only started investigating once lawmaker Lam Cheuk-ting helped bring the case to light.
A sit-in is also planned at Yuen Long MTR for the one-month anniversary of the attacks by a mob in white shirts.
Surveillance footage from North District Hospital in Sheung Shui appears to show two officers hitting the elderly man on the night of June 25.
The demonstrations have gone from protesting the fugitive bill to pressing for universal suffrage, and inquiries into alleged police brutality.
The assault in east Kowloon took place on Tuesday morning; no arrests have been made so far.
In her press conference, the Chief Executive did not comment on the public's demands for an independent commission into alleged police misconduct.
A non-cooperation movement on the MTR rail system and a rally held by secondary students are among further events scheduled this week.
Although the rally was only permitted to take place in Victoria Park, police did not condemn the demonstration and described the day as nonviolent.
The gathering is organised by the Civil Human Rights Front.