A prominent south Armagh republican and his brother are among four people being questioned by detectives about the murder of the IRA informer Mr Eamon Collins. It is understood the other two men being held are also brothers. Six vehicles were seized in the RUC operation.
No organisation has admitted responsibility for the murder, but the Provisional IRA is widely believed to have been involved. Mr Collins was beaten and stabbed to death near his home in Newry, Co Down, in January. His body was found on a country road.
The four men were arrested early yesterday at their homes in Dromintee. Sinn Fein condemned the arrests as "provocative harassment" and demanded the men's immediate release.
Meanwhile, a 39-year-old man is recovering in hospital after being shot in both legs in a so-called "punishment" attack in north Belfast. The Provisional IRA is believed to be responsible.
The man was walking in Stratheden Street, in the New Lodge area, when he was bundled into a car and driven to Flax Street, Ardoyne, where he was shot.
Mr Vincent McKenna, of Families Against Intimidation and Terror, said: "It is totally unacceptable that these human rights abuses continue to be carried out by an organisation that is supposed to be on ceasefire. How many cripples have to be created by the Provisionals before the government wakes up to its obligations to the victims of terrorist violence?"
A Sinn Fein councillor in north Belfast, Mr Mick Conlon, said such attacks were wrong. "I would advise people in the absence of a credible policing service to take their complaints to the administrators of restorative justice projects," he said.