* Prime Minister gives peacekeepers fresh mandate
* Peacekeepers accused of slow response to mosque attack
* Calls for disarmament as divisions intensify (Updates with new toll, gunfire, comment from government, U.N.)
By Serge Leger Kokpakpa
BANGUI, May 29 (Reuters) - Youths in the Central African Republic plundered a mosque in the capital and barricaded streets with burning tyres on Thursday in protest at an attack by Muslim gunmen on a church that killed about 15 people dead, witnesses said.
Gunfire rang out as U.N. peacekeepers attempted to clear the streets and few people ventured out because of the protests, which have heightened fears of further inter-religious violence and reprisals against the Muslims who remain in the city.
A Reuters witness saw youths from the mainly Christian "anti-Balaka" militias sacked the mosque in Bangui's Lakouanga neighbourhood. There were no casualties as the mosque was empty.
The country has been gripped by ethnic and religious violence for a year since Seleka rebels, who are mostly Muslim, seized power. The Seleka left power in January under international pressure and since then the anti-balaka militias have attacked on Muslims.
Those attacks have largely driven Muslims from the capital and other surrounding areas to the north and neighbouring countries, effectively partitioning Central African Republic, whose northeast is controlled mainly by Muslim rebel forces.
Thousands have been killed and about a million people displaced because of the conflict. More than 2.5 million people need humanitarian aid, a figure that represents more than half the population.
Thursday's protests were a response to an attack a day earlier in which gunmen sprayed bullets and hurled grenades at people sheltering at the Our Lady of Fatima church following a battle between anti-balaka militiamen and residents of the nearby Muslim neighbourhood of PK5.
Prime Minister André Nzapayéké condemned the attack, which he said killed 15 people, according to a provisional estimate. He gave international peacekeepers a mandate to clear out "criminal" elements from the city.
"In Bangui, some criminals mutilate their victims bodies savagely while others open fire on religious buildings with heavy weapons and massacre peaceful citizens in distress," he said in a statement.
U.N. Secretary-General Ban Ki-moon also condemned the church attack.
PEACEKEEPERS TOO SLOW
Sebastien Wenezoui, a leader of the anti-Balaka, accused international forces of abandoning the church to its attackers and singled out Burundian soldiers among the African peacekeeping force, called MISCA, as well as French soldiers who he said could have reacted faster to the initial onslaught.
"It's very sad. What hurts us most is that France is here to protect the civilian population. MISCA is there to protect the population but when we called the Burundians they didn't come," he told Reuters by telephone.
His comments were echoed by Catholic priest at the church Jonas Bekas who said peacekeepers were slow to respond to frantic calls he and other priests made from inside the church to say they were under attack.
The attackers entered the church compound when its defenders, members of the anti-Balaka, ran out of ammunition and once inside they appeared to target women and children, he said.
"If there's no disarmament we think they will attack us again," Bekas told Reuters, adding that he would remain at his post along with other priests to offer shelter to civilians.
"People need to be disarmed, especially the people who are killing other people," he said.
A spokesman for African Union MISCA peacekeeping force, Francis Che, denied the force had been slow to respond and said an investigation had been launched into what happened. (Additional reporting and writing by Matthew Mpoke Bigg in Accra; Editing by Angus MacSwan)