At least 44 people were killed in protests against Congolese President Joseph Kabila, including 37 killed by security forces and six police officers killed by demonstrators, Human Rights Watch (HRW) says.
The protests in the Democratic Republic of 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 manoeuvre 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.
Witnesses told Reuters earlier that at least two people were killed when armed men in uniform set fire to the opposition HQ.
Interior ministry spokesman Claude Pero Luwara told Reuters 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.
Congo has not had a peaceful transfer of power since independence from Belgium in 1960.
