The British government has held an emergency meeting after a string of crude explosive devices were sent to armed forces recruitment offices.
Prime Minister David Cameron chaired a meeting of the government's COBRA emergencies committee after suspect packages were sent to seven offices in southeast England.
Counter-terrorism police are investigating and army bomb disposal units have been sent to assist.
A source said the devices were "crude" in design but "could have caused injury".
An envelope was delivered to an office in Chatham and a package was found in Reading on Tuesday.
Sky News television, citing sources, said they were small but viable explosive devices.
Another was discovered on Wednesday in the army town of Aldershot, while four more were found on Thursday in Brighton, Canterbury, Oxford and Slough, the police's South East Counter Terrorism Unit said.
"The contents of the packages are suspicious in nature and will now be sent off for forensic examination," said Detective Superintendent Stan Gilmour.
"Even if the contents are determined to be a viable device they pose a very low-level threat and are unlikely to cause significant harm or damage.
A shopping centre in Slough to the west of London was temporarily evacuated, while cordons were placed close to all offices where packages were found.
"When a suspect package is reported we have a routine response which means we may need to evacuate the area if necessary until we can be sure it poses no threat to the public," Gilmour said.
"It is a necessary precaution until we know what we are dealing with."
A Ministry of Defence spokesman said security advice has been reiterated to its personnel, while the Royal Mail postal operator said it was co-operating with the police investigation.
Meanwhile, in an incident thought to be unrelated, a suspicious package was found during a routine vehicle search at the gates of Mildenhall air station in eastern England, one of the major US military airbases in Europe.
British bomb disposal units were called in and the item was determined to be a home-made firework.
A spokeswoman for the base said the device intercepted there "is not thought to be terrorist-related".
