Bomb disposal units have been called to deal with the packages, sent to at least six offices in southeast England, with police sources saying there was a "low-level" but "viable" threat.
Packages have been sent to offices in Aldershot, Chatham, Brighton, Oxford, Reading, Slough.
"Because suspicious packages were found at various locations yesterday, notes were sent out to all the army careers offices, as well as the Post Office, asking staff to be extra vigilant and look out for any suspect packages," said Detective Superintendent Stan Gilmour, of the South East counter terrorism unit.
The Queensmere Shopping Centre in Slough was evacuated, while cordons were placed close to all offices where packages have been found.
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A Ministry of Defence spokesman confirmed they were aware of ongoing security incidents in Brighton, Oxford and Slough involving armed forces careers offices.
"The relevant police authorities are investigating with military explosive ordnance disposal teams in support," he said.
"Security advice has been reiterated to our personnel.