Violence first flared on Monday after two 14-year-old boys were arrested and appeared in court, accused of a serious sexual assault on a teenage girl in the town.
The charges were read via a Romanian interpreter to the boys, whose lawyer told the court they denied the charge, the BBC reported.
One of a number of anti-immigration protests on Thursday was held in Portadown, 50km from the capital Belfast.
A large policing operation of officers in riot gear and armoured vans closed off a number of roads in advance.
Debris was strewn across streets and wheelie bins were set on fire.
Other protests passed off without major incident, including in Ballymena, the primary flashpoint of the first two nights of more intense violence, local media reported.
Paul Frew, a member of the regional assembly from Ballymena, said that while some people gathered on the streets again amidst a big police presence, it was much quieter and that heavy rainfall had helped keep people away.
"Hopefully we're through the worst of it," Frew, a member of the Democratic Unionist Party, told the BBC.
