Police say the teenage boy had been carrying spray paint and a ‘laser gun’ when he was arrested in Tseung Kwan O during a protest.
He was arrested on suspicion of unlawful assembly, possessing an offensive weapon and an instrument for unlawful purposes.
The boy is required to report back to the police in a month’s time.
The 13-year-old girl was arrested in Tuen Mun after a national flag was burnt near Tuen Mun Town Hall on Saturday.
She was arrested on suspicion of desecrating the national flag, and will have to report back to police later this month.