Three policemen in Bahrain were killed by a bomb explosion on Monday during confrontations between security forces and "rioters" in a Shiite village near the capital of Manana, the interior ministry said.
"Three police members died in a terror explosion in Daih while police were dispersing rioters," the Bahraini interior ministry said on Twitter, Al Arabiya News Channel reported.
The UAE's interior ministry said that an Emirati policeman was among those killed.
King Hamad bin Isa Al Khalifa called for a day of mourning on Tuesday to mark the officers' deaths and extended and ordered security forces to bring those responsible to justice.
"The King directed all competent state agencies to enforce the law on the instigators of the abominable terrorist blast and take all necessary measures to ward off such acts of terror on all levels," the Royal Court said in a statement carried by the official Bahrain News Agency.
The UAE's state news agency WAM identified the Emirati killed as 1st Lt. Tareq Mohammed al-Shehi. It said he was serving in Bahrain as part of the "Gulf Waves Force" deployed under a joint security cooperation agreement among Gulf Cooperation Council States.
Earlier, Bahrain's interior ministry said an officer and two policemen were "seriously" wounded in the village of Daih, on the outskirts of Manana, AFP reported.
Security forces had dispersed a "breakaway group of thugs who diverted from a funeral route in Daih to riot," the ministry added.
Witnesses reported hearing a blast in Daih during a confrontation between protesters and police, who fired tear gas and buckshot to disperse the crowds.
On Sunday, a policeman was wounded in a blast in the Shiite village of Akr al-Sharqi, said the interior ministry.
Bahrain's Shiites began anti-government protests in mid-Feb. 2011.
Dozens of people have been killed since protests began, according to the International Federation for Human Rights.
In 2013, authorities increased the penalties for those convicted of violence, introducing the death penalty or life sentences in cases which resulted in deaths or injuries, according to AFP.