A primary school caretaker was convicted yesterday of masterminding a letter-bomb campaign which left eight people injured and struck fear into many more. It took a jury at Oxford Crown Court one hour to find Miles Cooper, 27, guilty of all 11 charges against him.

Recipients who opened Cooper's padded envelopes were showered with glass fragments or nails, the four-day trial heard. Cooper said he sent the seven letters, five of which exploded, in protest at Britain's "authoritarian" government.

He said he resorted to violent means as his peaceful protests against measures imposed by Tony Blair's administration had fallen on deaf ears.

Offices in London, Berkshire, Oxfordshire, Kent, Birmingham and Swansea were targeted by Cooper in January and February.

When police swooped on his home in Cherry Hinton, near Cambridge, they discovered a "bomb factory" in his bedroom with three more devices ready to send.

Cooper said: "The overall goal was to shut down certain departments in certain buildings and ultimately to highlight my cause."

He denied eight counts of causing bodily injury by means of an explosive substance, two counts of using an explosive substance with intent to disable, one count of making explosives and one alternative count of possessing such a substance.

He did not deny sending the letters to three forensic-science laboratories, a computer company, an accountancy firm, the DVLA and a residential address, but did deny intending to cause any injury.

Adjourning sentencing until tomorrow, Judge Julian Hall said: "They are verdicts with which I wholeheartedly agree."

Michael Wingfield, one of Cooper's victims, who was in court to witness his conviction, said: "I'm as happy as I can be, I suppose."

Earlier, Cooper said in evidence that his anger at the authorities intensified when his father, Clive, was unable to get DNA samples removed from the police database after he was cleared in 2003 of assault.

The treatment of anti-nuclear protesters further angered him, as did that of Walter Wolfgang being thrown out of the 2005 Labour Party conference for heckling Jack Straw, then the Foreign Secretary, he said. "It became more and more obvious that the Government was not going to listen to peaceful protesters," Cooper said.

Cooper accepted people were likely to suffer cuts and bruises when opening the packages but he said he intended no serious harm.

"The overall goal was to shut down certain departments in certain buildings and ultimately to highlight my cause," he said. "I am genuinely ashamed of what I've done."