Two plastic bottles filled with an unspecified acid were thrown from a building in the Mong Kok shopping district onto the street below, police said.
Forty-six people suffered burns. Some victims rushed to find water to wash the burning liquid from their faces, eyes and hands, local media said.
The victims, aged 14 to 53, were released from hospitals the same day after receiving treatment.
"The difficulty we have is twofold: One is not knowing the motive, and secondly, not knowing who is responsible," Paul Grove, an assistant district police commander, told The Associated Press. "But we are treating the case very seriously."
Among the leads police are investigating is a message posted on the Internet claiming responsibility for the attacks, authorities said.