Bahrain has executed three Shia men who were convicted of killing three police officers in a bomb attack in March 2014, the authorities say.
They were killed by a firing squad on Sunday, state news agency BNA reports.
Human rights officials say there are serious concerns that evidence may have been obtained under torture.
But the Bahraini government, which rarely carries out executions, has said the decision was taken "in accordance with international law".
The executions of the three men, who Bahraini officials say were part of the listed terrorist group Saraya al-Ashtar, are the first since a 2011 uprising, led by the Shia majority, calling for greater political rights.
The Sunni-ruled kingdom has escalated a crackdown on its Shia critics over the past year, including revoking the citizenship of the country's most prominent Shia Muslim cleric.
The UN's special rapporteur on extra-judicial killings, Agnes Callamard, condemned the executions in a tweet saying: "Torture, unfair trial + flimsy evidence: these are extrajudicial killings."
Maya Foa, head of the UK-based human rights group Reprieve, said: "It is nothing short of an outrage - and a disgraceful breach of international law - that Bahrain has gone ahead with these executions."
The death sentences handed to the three men, she added, "were based on 'confessions' extracted through torture, and the trial an utter sham".
But a statement from the government said evidence "included in part fingerprints on the IEDs, phone records which confirmed the locations of the three men (e.g., at the time of the attacks) and wide-ranging witness statements".
The statement added that the decision to execute the men met "all nine of the United Nations Safeguards" - including "clear and convincing evidence" which left "no room for an alternative explanation of the facts".
Abbas al-Samea, Sami Mushaima and Ali al-Singace were convicted of a bomb attack that killed three policemen, including one officer from the United Arab Emirates, nearly three years ago.
They were executed a week after their death sentences were upheld by a high court.
The executions are the first to be carried out in Bahrain in more than six years, according to Reprieve.
"This is a black day in Bahrain's history. It is the most heinous crime committed by the government of Bahrain and a shame upon its rulers... This act is a security threat to Bahrain and the entire region," Sayed Ahmed Alwadaei of the Bahrain Institute for Rights and Democracy told Reuters news agency.
Protests erupted in several Shia villages over the news of the executions. When demonstrators blocked roads with burning tyres, police responded with tear gas.
A police officer was wounded when his patrol came under fire on Saturday in the Shia village of Bani Jamra, to the west of the capital Manama, the interior ministry said.