At least 17 people, including three policemen, were killed when Congolese police and anti-government protesters clashed in the capital Kinshasa on Monday during a march against President Joseph Kabila and what the protesters see as his bid to extend his mandate.
The protest, attended by thousands, came at a time of growing local and international pressure on Kabila to step down when his term of office legally ends in December.
The opposition accuses him of plotting to extend his tenure by delaying elections that were supposed to be held in November until at least next year. His supporters deny this.
"Officially, we have 17 dead in Kinshasa: three police and 14 civilians," said Interior Ministry spokesman Claude Pero Luwara.
Earlier, a Reuters witness saw a crowd burning the body of a police officer in the Kinshasa suburb of Limete in an apparent act of retaliation for police gunfire.
Angry crowds tore down photographs of Kabila, chanting in French: "it's over for you" and "we don't want you".
Georges Kapiamba, director of the Congolese Association For Access to Justice, a local non-governmental organisation, said that security forces shot dead twenty-five protesters.
Rights groups reported dozens of arrests of protesters and journalists in the capital as well as in Goma and Kisangani, where anti-government marches also took place.
A government spokesman confirmed the detention of opposition leader Martin Fayulu, who suffered a head injury during the march.
By mid-afternoon, most protesters had been dispersed and the streets in the normally bustling city centre were quiet.
Monday's march was sparked by anger over a decision by the election commission last week to petition the constitutional court to postpone the next presidential vote.
The vast, mineral-rich central African state has never seen a peaceful transition of power. Western observers and donors fear that growing political instability could mushroom into armed conflict in a country plagued by militias, especially in its lawless eastern regions.
Millions of people died in regional wars in Congo between 1996 and 2003 that drew in armies from half a dozen countries.
Opposition leaders had planned to file an official petition against the decision before the clashes began. The police said in a statement that officers had been warned to use restraint, adding that it would punish those who do not comply.
Dozens of people died in similar protests against Kabila last year. Human Rights Watch said in a statement on Monday that the clashes were preceded by intensified government repression.
"Today's march shows that the security forces have not switched their tactics and are still clamping down on anyone opposed to Kabila," said Ida Sawyer, senior researcher at Human Rights Watch.
"People want their constitution to be respected and are willing to risk their lives to make sure that happens." She added that three children were shot in Goma during the clashes.
French Foreign Minister Jean-Marc Ayrault on Monday described the situation in Congo as "extremely worrying and very dangerous", adding that European nations will discuss the possibility of imposing sanctions.
The United States has already threatened sanctions against political figures in Congo over electoral delays. A senior adviser to Kabila has pleaded to Washington not to.
The U.S. embassy said on its Twitter feed that it was "outraged" by the harassment of its special envoy for the Great Lakes Region of Africa, Thomas Perriello, at Kinshasa's airport.
Congo's "Presidential Majority", as Kabila's supporters call themselves, had accused Perriello, in a Sunday statement, of jeopardising talks aimed at resolving Congo's political crisis.
(Additional reporting by Benoit Nyemba; in Kinshasa and Nellie Peyton in Dakar; writing by Emma Farge; editing by Tim Cocks, Larry King)
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