(Restores dropped word 'credibility' in last paragraph)
KINSHASA, Dec 31 (Reuters) - Security forces killed at least seven people in the Democratic Republic of Congo on Sunday during protests against President Joseph Kabila's refusal to step down from office, United Nations peacekeepers said.
A police spokesman said three people had been killed in altercations with security forces in the capital, Kinshasa, and that two of the deaths were being investigated.
Catholic activists had called for protests after Sunday worship, one year after Kabila committed to holding an election to choose his successor by the end of 2017 - an election that has now been delayed until December 2018.
The delay has fuelled suspicions that Kabila will try to remove constitutional term limits that forbid him to run again, as presidents in neighbouring countries have done. That in turn has raised fears that Congo will slide back into the kind of civil war that killed millions at the turn of the century.
Florence Marchal, spokeswoman for the U.N. peacekeeping mission in Congo, said security forces had shot dead at least seven people in Kinshasa. Another person was killed in a protest in the central city of Kananga, she said, although the cause of death was not yet clear.
U.N. observers documented at least 123 arrests across the country and a number of serious injuries, Marchal added.
In tweets, the U.N. human rights office in Congo condemned "the use of force against peaceful demonstrators" and the "violent suppression of fundamental rights and freedoms by security forces".
Pierrot Mwanamputu, the national police spokesman, said one person had died in Kinshasa when security forces were attacked by young men, some of them carrying firearms and bladed weapons. He said the other two deaths occurred during an altercation and that the police had opened an investigation.
At the Paroisse Saint Michel Catholic church in Kinshasa's Bandalungwa district, security forces fired teargas into the building, creating panic, opposition leader Vital Kamerhe, who was present at the mass, told Reuters.
The police had banned the demonstrations and said that all gatherings of more than five people would be dispersed to ensure public order. Across Kinshasa, police and soldiers searched vehicles and checked passengers' identifications.
Authorities on Saturday ordered all internet and SMS services cut until further notice.
At the Notre Dame du Congo cathedral in Kinshasa's Lingwala district, where opposition leader Felix Tshisekedi was attending mass, dozens of police and soldiers blocked the path of more than 100 opposition supporters as they prepared to try to march.
But Tshisekedi, who had backed the activists' call to march, left the church in a vehicle, spurring angry shouts from the crowd, which said he was abandoning them.
At another church in the working-class district of Barumbu, a Reuters correspondent saw a few dozen police officers using teargas and stun grenades against some 300 churchgoers, who waved bibles and sang religious songs as they tried to march.
Some 40 percent of Congo's population is Roman Catholic and the Church, which has mediated in the dispute with Kabila, enjoys some public credibility, even though its leadership has not formally backed the protests.
(Writing by Aaron Ross; Editing by Larry King and Kevin Liffey)
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