A 76-year-old man was arrested in Portsmouth after allegedly concealing his identity following the introduction of a ban on face coverings at protests.
Hampshire Police activated new powers under the Crime and Policing Act at 10:00 BST, prohibiting protesters from wearing face coverings in designated areas.
Separately, a 33-year-old Portsmouth man was detained on suspicion of obstructing a highway and criminal damage during demonstrations.
A 16-year-old boy was arrested on suspicion of violent disorder linked to a protest at Eastney Marina on Sunday.