At least 27 people, including three police officers, were killed in clashes between protesters and police in Democratic Republic of Congo earlier this week, Human Rights Watch said on Thursday.
The majority of the deaths were in the capital, Kinshasa, where members of the separatist sect Bundu dia Kongo (BDK) marched against President Joseph Kabila and attacked a prison on Monday.
BDK is just one of many sources of opposition of Kabila that have threatened to plunge the vast, mineral-rich central African country into chaos since the president refused to step down when his mandate expired in December.
"Prompt and impartial investigations are needed to determine who is responsible for the deaths .... Violence by protesters or excessive use of force by security forces should not be tolerated and those responsible should be held accountable," HRW Central Africa director Ida Sawyer said.
Congo security forces shot into crowds to disperse the protesters in Kinshasa, killing 11 BDK members and 10 bystanders, Human Rights Watch said in a statement.
Others were killed in similar clashes in the southwestern cities of Matadi and Muanda, it added.
Congo's police spokesman said on Wednesday that 19 people had been killed in total, including 17 BDK members and two police.