Crowds gathered outside Southampton's police station on 2 June before moving to the Portswood area.
Two Reform councillors quit weeks after being elected, triggering the by-elections.
Drivers experience disruption for about 14 hours as a result of a series of lane closures.
Officers want to break up anti-social behaviour by groups across the weekend.
Henry Nowak, 18, was handcuffed as he lay dying after being stabbed by Vickrum Digwa in December 2025.
"It's days like this that help the family keep going," says Lucy Ross, Henry's mum.
It follows members of the faith reporting hostilities since murderer Vickrum Digwa was jailed.
Henry Nowak's killer is seen lying to police in bodycam video released after a request from the BBC.
The Southampton student died from his injuries, while handcuffed on the ground by Hampshire police officers
