At least 44 people were killed in protests against Democratic Republic of Congo President Joseph Kabila, including 37 killed by security forces and six police officers killed by demonstrators, Human Rights Watch (HRW) said on Tuesday.
The protests in the DR Congo that started on Monday followed a decision by the election commission to seek to postpone the next presidential vote, which was due to be held in November.
Kabila is barred by constitutional term limits from running again and his opponents say the election delay is a maneuver to keep him in power. The president's allies deny this and say he will respect the constitution.
HRW Africa researcher for the New York-based group, Ida Sawyer, said 17 people were killed overnight and on Tuesday, while 20 had been killed on Monday.
Her statement was based on what she said were credible reports.
"Most were killed when security forces fired on crowds of protesters. Others died when the security forces burned down opposition party headquarters last night," she said.
Witnesses told Reuters earlier that at least two people were killed when armed men in uniform set fire to the opposition HQ.
"We've also received credible reports that protesters have killed at least six police officers and a (ruling party) PPRD supporter and they have also burned and looted several shops and police stations," Sawyer said.
Interior ministry spokesman Claude Pero Luwara said the death toll stood at 17 and said Human Rights Watch's statement was a "typical" exaggeration by the group.
The ministry earlier said three people who died were policemen.
UN human rights spokesman Rupert Colville told reporters in Geneva that nearly 200 people were believed to have been arrested on Monday and the UN received reports of excessive use of force by security forces.
DR Congo has not had a peaceful transfer of power since independence from Belgium in 1960.