Counter-terrorism police are investigating reports of suspect packages being found at army careers offices.
Packages have been found in Canterbury, Oxford, Slough and Brighton, while similar packages were found earlier in the week in Aldershot, Reading and Chatham, the south east counter terrorism unit said.
Police officers consider the situation a "low-level threat" but "viable", sources said.
Ministry of Defence bomb disposal units have been called, although this is routine with any suspect package.
Detective Superintendent Stan Gilmour, of the south east counter terrorism unit, said: "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."
Queensmere Shopping Centre, in Slough, was evacuated, while cordons were placed close to all offices where packages have been found.
A Ministry of Defence spokesman said: "We can confirm that we are aware of security incidents ongoing in Oxford, Slough and Brighton involving armed forces careers offices.
"The relevant police authorities are investigating with military EOD (explosive ordnance disposal) teams in support and any further inquiries should be directed to the police.
"Security advice has been reiterated to our personnel."