A Bahrain court handed 15-year jail sentences to 11 defendants after convicting them of "manufacturing bombs for terror purposes", the official BNA news agency reported.
The High Criminal Court jailed another defendant for five years on similar charges at Tuesday's hearing, BNA quoted prosecutor Nawaf al-Awadhi as saying in a statement.
Bahrain, home to the US Fifth Fleet, remains deeply divided three years after a quashed Shiite-led uprising, with persistent protests sparking clashes with police, scores of Shiites jailed on "terror" charges and reconciliation talks deadlocked.
The 12 defendants were charged with "possessing weapons, ammunition and explosives as well as manufacturing bombs for terror purposes," said Awadhi.
The group smuggled ammunition and bomb-making materials into the country and hid them on a farm, the statement said.
Signup to our newsletter and follow us on Facebook and Twitter!
Among the weapons seized were five Kalashnikovs and five revolvers in addition to explosives and ammunition.
They planned to target members of the security forces and public figures to "spread chaos and terror" in the Sunni-ruled kingdom.
According to the statement, some of the defendants had confessed to their involvement in the plot while others of the group remain at large.
The statement did not give details on the number of defendants tried in absentia.
The authorities last year increased the penalties for those convicted of violence, introducing the death penalty or life sentences in certain cases.
The International Federation for Human Rights says at least 89 people have been killed in Bahrain since the uprising began in February 2011.