Human Rights Watch criticised "failures" in Bahrain's justice system, saying it severely punishes pro-reform protesters while offering impunity to abusive security personnel.
"A police officer in Bahrain who kills a protester in cold blood or beats a detainee to death might face a sentence of six months or maybe two years," said HRW deputy Middle East director Joe Stork in a 64-page report, released Thursday.
Meanwhile, "peacefully calling for the country to become a republic will get you life in prison."
Scores of Shiites were rounded up following a crackdown on protesters against the ruling Sunni dynasty in March 2011, and many have been tried and jailed.
The kingdom has been widely criticised over the and for the alleged mistreatment of detainees.
The report alleges that the kingdom's courts maintain a "highly repressive political order."
In November 2011, the Bahrain Independent Commission of Inquiry found that security forces had used "excessive force" and tortured detainees in its month-long crackdown on dissent.
Among other recommendations, it called for reforms in the police and justice systems.
HRW's report criticised "the stark contrast between prosecutions of serious human rights violations by security personnel on the one hand and prosecutions for 'crimes' based on speech and peaceful assembly-related activities on the other."
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"Bahrain's problem is not a dysfunctional justice system, but rather a highly functional injustice system," said Stork.
Last October, an appeals court cut from seven years to three the jail terms of two policemen convicted of torturing a Shiite detainee to death.
That same month, the jail term of a policeman convicted of shooting dead a protester in 2011 was also reduced from seven years to three.
The policeman was convicted of shooting a protestor with birdshot, but the court said the killing was not premeditated.
HRW said Bahrain's allies in London, Washington, and Brussels have failed to press the government to "take serious steps to hold security forces accountable for abuse, or to call openly for the release of high-profile political prisoners."
Authorities in the Gulf kingdom, home to the US Fifth Fleet, have increased penalties for those convicted of violence, introducing the death penalty or life sentences for certain cases.
On Monday, a court jailed four people for life for blowing up a car in an attempt to disrupt the F1 Grand Prix race in 2013. A fifth man was given 10 years.
In March, prosecutors said that four Shiites who "confessed" to taking part in an attack that killed three policemen in Bahrain could face the death penalty if convicted.
However, death sentences are usually commuted to life imprisonment -- 25 years.
The International Federation for Human Rights says at least 89 people have been killed in Bahrain since the uprising began in February 2011.