GENEVA — The United Nations on Tuesday expressed alarm over mounting repression in the Democratic Republic of Congo after security forces fired on anti-government protesters, killing at least six people, and attacked a United Nations official monitoring the protests.

The violence erupted Sunday during protests in the capital, Kinshasa, and other major cities calling on President Joseph Kabila to step down and hold free elections. Mr. Kabila was to step down at the end of 2016 at the end of his second term, as constitutionally mandated. But he refused to do so.

United Nations human rights monitors in Kinshasa had verified the deaths of six people but were investigating reports that four more people had been killed and believed the number of fatalities could rise, said Ravina Shamdasani, a spokeswoman for the United Nations human rights office in Geneva. Ms. Shamdasani also said that 68 people were injured and 121 people were arrested during the demonstrations.

During the protests, Congolese security forces attacked a United Nations human rights official who was monitoring the events, throwing him to the ground and kicking and punching him even though he was wearing a blue vest clearly identifying him as a member of the United Nations human rights office.