Bangui - Four people were killed and over 20 wounded when grenades were thrown into a concert in the capital of the chronically unstable Central African Republic, while revenge attacks later left another three people dead, the UN peacekeeping force and a local official said Sunday. In the first serious incident of violence in Bangui this year, the assault late Saturday saw two unidentified attackers hurl grenades at a cafe called "On the Crossroads of Peace" during a performance, said Herve Verhoosel, spokesman for the UN peacekeeping force MINUSCA.CAR's minister for public security, Henri Wanzet, told state radio that four people had died in the attack and around 20 were injured. An official probe has been opened, Wanzet said. The cafe is located near the PK5 majority Muslim district of Bangui, once a Muslim rebel bastion, and now home to several armed groups. Even though relative peace has been restored in Central Africa following the civil war that erupted in 2013, inter-communal tensions remain.Among those injured in Saturday's attack were several musicians from the group that was performing, Verhoosel said.A doctor at the hospital where the injured were rushed for treatment said 21 people had been admitted.Seven others were taken to another health centre, a local official with Doctors Without Borders (MSF) medical charity said.A local public figure said three people were killed later in the night in revenge attacks -- two of whom had their throats cut and one who was beaten to death.'Innocents' "They were two motorcycle-taxi drivers and a young man who was walking in the neighbourhood," the man, who asked not to be identified, told AFP. "They are innocents".He appealed for calm in the neighbourhood where sporadic gunfire could be heard and armed members of "self-defence" groups patrolled. An armoured car carrying MINUSCA troops was stationed at the entrance to PK5.An AFP correspondent saw three bodies taken to the morgue from a local mosque.A witness who did not give his name said the attackers at the cafe had launched four grenades.In a video posted on his Facebook page, the concert's singer Ozaguin - dubbed the king of Central African rumba - confirmed six of his fellow musicians had been injured.He said he was surprised that he was not himself injured by grenade fragments."Ozaguin came to perform here to ensure that all Central Africans - Muslims and Christians - can find some social cohesion," the cafe's owner, Issiakou Guymba, told AFP."An unidentified group came on a motorcycle-taxi, they threw one or some grenades in the middle of the crowd," he said.Violence "It makes us lose hope, when people come to cause panic like that among the population."Mired in poverty but rich in diamonds and other minerals, CAR has been battered by a conflict between rival militias that began after then-president Francois Bozize was overthrown in 2013.Under a UN mandate, the former colonial power France intervened to push out the Muslim Seleka rebels who had taken over, and the UN launched a peacekeeping mission in 2014.But the country remains chronically prone to violence, with armed groups controlling most of the country.Fighting between armed groups around the town of Batangafo in northern CAR left at least two dead and 10 wounded at the end of last month.