In a social media post, the force noted that organisers had said they would bar anyone who violated the law, or official rules, from joining the race.
The force said officers would collect evidence at the scene according to the situation, and take further action if necessary.
Police said their counter terrorism response unit and railway response team had conducted high-profile patrols near the race’s route.
Officers also searched various MTR stations for dangerous items that might be hidden.
Sunday’s race will be the first mass participatory sporting event since the national security law was implemented last year.