Ordinary workers have been forced into the "front line" after two letter-bombs were handled by them, it has been claimed.
Yesterday viable devices in packages addressed to senior police officers were intercepted at two Royal Mail sorting offices.
The first device was addressed to PSNI Chief Constable Matt Baggott at Knock police headquarters and was detected in the Royal Mail Mallusk sorting office.
The second was found yesterday morning in a Lisburn postal delivery office and is thought to have been addressed to Chief Inspector Jon Burrows in Londonderry.
Assistant Chief Constable Will Kerr said dissidents were believed to be behind the attacks.
South Antrim DUP MP William McCrea said the attacks have resulted in ordinary workers being put in the front line.
"There has to be protection put in place for the workers in the sorting offices because we have to be conscious that they are being put in the front line here." ACC Kerr said: "Undoubtedly there were dissident groupings who wanted to kill police officers and others.
"Fortunately we are not dealing with fatalities this morning. But we could have very easily been dealing with somebody being very seriously hurt or killed.
"We need community support and information to help us tackle these groups effectively and that is what we are asking," he added.
Secretary of State Theresa Villiers condemned the bombs, calling them "callous and reckless".
Justice Minister David Ford commended the vigilance of the sorting staff at Royal Mail and said that those responsible for the devices "have absolutely no place in a normal society".
Lisburn SDLP councillor Pat Catney said he was disgusted.
He said: "It's just heartbreaking and soul-destroying when we are all working here as a community to make where we live a better place and some misguided people for whatever reason go about just disrupting daily lives."
A Royal Mail spokeswoman said there would be a knock-on effect on deliveries as a result.
On Thursday night a couple with four young children escaped injury after a pipe-bomb was put through the letterbox of their home in Carlisle Parade in north Belfast.
Local residents expressed anger at the attack.
Last week two schools and several homes were evacuated in Lurgan after a device was found. A security alert in Holywood was declared a hoax. On Tuesday a pipe-bomb was thrown at a police car in Derry, and in Newtownabbey on Wednesday a security alert was declared a hoax. Police are appealing for anyone with information relating to the attacks to come forward.