Police are investigating an explosion in which two 12-year-olds and a two-year-old were said to have suffered minor injuries from flying debris in Lurgan, Co Armagh.
There were appeals for calm as a major loyal order parade took place in Northern Ireland.
The first leg of the Apprentice Boys parade in Londonderry was held without major incident after a protest by dissident republicans was cancelled.
But in Lurgan an explosion in a wheelie bin led to three injuries with police now investigating reports of a second suspicious object.
Earlier nationalists staged a silent protest at a parade linked to the Derry march in the Ardoyne area of north Belfast, where there were riots after an Orange Order parade last month. But it too passed off without incident.
The relative calm raised hopes that violence can be avoided when the final leg of the main Derry parade takes place later today.
Sinn Fein president Gerry Adams welcomed the fact that events had remained peaceful in north Belfast.
He added: "I hope that the peaceful scenes this morning continue for the rest of the day in Derry and I would appeal to people to take their example from the behaviour of the community in north Belfast this morning."