Election-related violence in Democratic Republic of Congo has already killed 18 civilians, a rights group said Friday, amid fears that fresh unrest could erupt over alleged fraud.

Monday's polls in the restive central African country were rocked by rioting at polling stations and deadly attacks in the flashpoint southeastern city of Lubumbashi, after a campaign marred by police crack-downs on opposition rallies and clashes between supporters of rival parties.

In the most extensive report yet on violence around the polls, Human Rights Watch (HRW), which examined election-related killings from the last day of campaigning on November 26 to the vote two days later, said at least 18 civilians had died, mostly shot dead by security forces in the capital Kinshasa.

"Elections don't give soldiers an excuse to randomly shoot at crowds," Anneke Van Woudenberg, Human Rights Watch senior Africa researcher, said in a statement.

"The authorities should immediately suspend those responsible for this unnecessary bloodshed and hold them to account."

HRW said at least 12 opposition partisans and bystanders were shot dead Saturday as the Republican Guard, the former presidential guard, quashed a rally by supporters of President Joseph Kabila's chief rival, Etienne Tshisekedi, who had gathered around Kinshasa's main airport to welcome him for a campaign rally.

Victims included a 27-year-old mother of five and a 22-year-old man who were both shot in the head while walking home from work around the airport, the group said.

"They began shooting at everyone in the crowds on the side of the road," a 21-year-old pregnant woman told HRW. "I tried to run, but the soldiers shot me in the foot."

HRW said other civilians were killed and wounded during clashes between rival political parties, attacks by armed groups and mob violence. At least 100 civilians were seriously wounded in election-related violence, it said.

Government spokesman Lambert Mende told AFP he would need to examine the report before responding.

The HRW toll is substantially higher than figures released by officials. Authorities in the southeastern province of Katanga had said 10 to 11 people died, including two police, in apparent separatist attacks in the restive city of Lubumbashi on election day.

Kinshasa police had said two people died in Saturday's violence around the airport, one who was hit in the head with a stone and the other in unclear circumstances.

Preliminary results from the presidential race, which pitted Kabila against a divided opposition field of 10 challengers, are expected Tuesday, while preliminary results from the parliamentary polls, in which 18,864 candidates are vying for 500 national assembly seats, are due January 13.

Human Rights Watch echoed the appeals for calm that have come from Western countries, international election observers and the UN peacekeeping mission in DR Congo.

The week-long political vacuum before the first official results of only the second elections since back-to-back wars from 1996 to 2003 has filled with tensions amid a swirl of fraud charges.

Observers have reported attempted ballot box stuffing and masses of voters being turned away from polling centres, among other irregularities.

Three opposition candidates have called for the vote to be annulled, accusing Kabila of massive fraud.

The ruling coalition has meanwhile accused Tshisekedi's party of plotting to spark post-poll violence.

Presidential Majority secretary general Aubin Minaku charged the party, the Union for Democracy and Social Progress (UDPS), with hatching "insurrectionist schemes", and vowed the government would crack down on any unrest.