Post office staff and military recruiters have been told to be "extra vigilant" and be on the lookout for further suspect packages after bombs were sent to Army offices in England.
Downing Street said the explosive packages bore "hallmarks of Northern Ireland-related terrorism". The seven bombs were described as small and crude.
They are said to have contained flammable black powder capable of causing burns.
Two were posted from the Republic, with the rest delivered from different locations in Britain.
Deputy First Minister Martin McGuinness condemned those who continued to engage in violence, saying "their futile acts must be condemned".
DUP deputy leader Nigel Dodds condemned those behind the bombs, saying: "We are moving forward and no one wants to go back to the bad old days."
Justice Minister David Ford said: "I unreservedly condemn the actions of those behind these incidents. They are not supported by the vast majority of people in Northern Ireland who want to live in a peaceful society."
Sinn Fein MLA Pat Sheehan said: "Thankfully no one was hurt because of these devices.
"Those behind sending them need to seriously look at what they are doing."
Prime Minister David Cameron chaired a meeting of the Government's Cobra committee on Thursday to discuss the threat.
Four suspected explosive devices were discovered at Army careers offices in Oxford, Brighton, Canterbury and the Queensmere shopping centre in Slough on Thursday, anti-terror police said.
One package was found in Aldershot in Hampshire on Wednesday, while two were found on Tuesday at the forces careers office in Reading, Berkshire, and the Army and RAF careers office in Chatham, Kent.
STORY SO FAR
Last October dissidents sent a series of letter bombs. One device was sent to Stormont Castle in Belfast and addressed to Secretary of State Theresa Villiers. Another was delivered to the PPS in Londonderry while two explosive packages, one addressed to Chief Constable Matt Baggott and the other to a senior commander, were intercepted.