An officer from the Police Tactical Unit was taken to Ruttonjee Hospital after being attacked when police closed a traffic lane during yesterday's march.
A police spokeswoman said the officer fell to ground while he was on duty near Thomson and Fleming roads.
Jackie Hung Ling-yu, from the Civil Human Rights Front, said the incident was probably related to a police decision to cut the number of traffic lanes from three to two.
Lawmaker and marcher Kwok Ka-ki said he believed the officer had been injured by the equipment he was carrying on his belt. He said no struggle had taken place and he had been told by witnesses the incident was caused by the decision to cut the lanes.
Ms Hung said the Front had sent its sympathy to the officer. Police said no one was arrested. No one saw who the attacker was. The officer was sent home last night.