By Kenny Katombe and Benoit Nyemba
KINSHASA (Reuters) - 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) said on Tuesday.
The protests in the Democratic Republic of Congo started on Monday after the election commission decided to try to postpone the next presidential vote, due in November.
Kabila's opponents say the proposed delay is a maneuver to keep the president in power although ruling party politicians deny this. Kabila is barred constitutionally from running for a third term and his allies say he will respect the constitution.
With the unrest forcing schools to close and halting public transport in the sprawling riverside capital, the United Nations expressed fears the situation could worsen.
Overnight, several people were killed when security forces burned down the headquarters of the main opposition party, the Union for Democracy and Social Progress (UDPS), and attacked buildings of other opposition parties, HRW Africa researcher for the New York-based group, Ida Sawyer, said.
Seventeen people had been killed overnight and on Tuesday, while 20 had been killed on Monday, she said.
"Most were killed when security forces fired on crowds of protesters," she said.
"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.
The fire - a criminal act - started between 2 a.m. and 4 a.m. (0100-0300 GMT), said opposition leader Felix Tshisekedi, a UDPS official and son of veteran opposition figure Etienne Tshisekedi, who lost to Kabila in a 2011 presidential run-off after five decades of active involvement in Congolese politics.
Congo has not had a peaceful transfer of power since independence from Belgium in 1960.
"We have on our hands seven dead and two people who were amputated: one at the arm and another at the leg, in the fire at our headquarters in Limete. There are also multiple people wounded at the hospital," Felix Tshisekedi told Reuters.
Tshisekedi, who has called for further protests, said: "We won't live with this barbarity. The people are angry." Witnesses said calm has returned to central Kinshasa by Tuesday afternoon, though there were reports of clashes in the suburbs.
Government spokesman Lambert Mende condemned the attack on the UDPS but denied security forces were involved.
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 had earlier said three of those killed were policemen.
Kabila took power in 2001 when his father was assassinated but must step down in December.
His opponents fear he may follow the example of leaders in other African countries such as Burundi and Rwanda and change the constitution to extend his rule.
Tom Perriello, U.S. Special Envoy for the Great Lakes region, said Washington would hold all sides accountable for their actions during the latest violence and that the government could easily have taken steps to defuse tension.
"Unfortunately, the pattern over the last year has been to dramatically reduce that open political space ... We have seen a crackdown on the rights and freedoms of Congolese people," he told a news conference in New York.
"We remain ready to impose additional targeted sanctions on individuals who have been involved in abuses or violence," said Perriello, who was verbally harassed and obstructed at Kinshasa airport as he left on Sunday.
Adding to the mix of disapproval, Congo's powerful Catholic church temporarily suspend its participation in talks between the government, some opposition parties and civil society groups over the election timetable, local media said.
Nearly 200 people were believed to have been arrested on Monday and the U.N. received reports of excessive use of force by security forces, U.N. human rights spokesman Rupert Colville told reporters in Geneva.
Congo is Africa's top copper producer but many Congolese people have yet to benefit from the country's mineral wealth.
(Additional reporting by Amedee Mwarabu in Kinshasa, Tom Miles in Geneva and Emma Farge, Tim Cocks and Nellie Peyton in Dakar; Writing by Matthew Mpoke Bigg; Editing by Louise Ireland)