Mass killings have been carried out by both sides of the conflict in Ivory Coast, according to the campaign group Human Rights Watch.

Their report documents a trail of death and destruction carried out by rebel forces who have swept through the country and are now fighting on the streets of Abidjan to secure the presidency for Alassane Ouattara.

As Ouattara, backed by the UN and the international community, edges closer to victory, the Guardian has uncovered evidence of atrocities committed by the forces acting in his name. Refugees who scrambled through the rainforest to safety in neighbouring Liberia have described children being burned alive during rebel attacks and bodies littering the streets.

HRW is calling for an investigation into massacres carried out by both the rebels and those loyal to the defiant president, Laurent Gbagbo, who refused to give up power after losing the presidential election in November.

Hundreds have been killed by forces loyal to Ouattara, according to HRW's report. It found that summary executions of perceived Gbagbo supporters had taken place, and reported accounts of mass rape. Matt Wells, HRW's Ivory Coast researcher, said: "In village after village, Ouattara's forces terrorised civilians perceived as supporting Gbagbo, killing hundreds and raping dozens more. In committing to move Ivory Coast out of its longstanding crisis, Ouattara must ensure that the perpetrators of these heinous crimes are brought to justice."

Pro-Gbagbo forces are also accused of having carried out atrocities, killing more than 100 presumed Ouattara supporters as rebels advanced.

The Guardian spent a week travelling in the border region between Ivory Coast and Liberia, hearing tales of savage attacks on civilians. It also encountered what is emerging as a recurrent aspect of the violence in Ivory Coast: the use of mercenaries from Liberia, believed to have been recruited by both sides in the conflict.

Crouching in the bushes along the banks of the river that separates Liberia from Ivory Coast, two young Liberian men in filthy clothes and flip-flops agreed to a recorded interview after a small payment was made. They described how they had just returned home from a nine-day operation with pro-Ouattara rebels, where they said they were told to kill "anyone and everyone".

They described barbaric scenes in which they surrounded villages in the west of Ivory Coast and, armed with machetes, killed everyone they saw. "The town we entered first, most of the people were on the road. We killed them, just cutting them with our machetes," they said.

One of the towns they claim to have attacked was Blolequin. UN investigators said yesterday they had found more than 100 bodies in Blolequin and surrounding towns. Some appeared to have been burned alive and others had been thrown into a well. The UN believes Liberian mercenaries may have been responsible.

Toulépleu is another town the two mercenaries say they attacked, and where HRW has uncovered evidence of mass killings. One mercenary said: "There are so many bodies in Toulépleu. A digger came from Danane to bury the bodies. There was no way for cars to go over there because of the bodies on the ground. It stank."

Now in the safety of a transit camp in Liberia, refugees fleeing from Toulépleu spoke of the horrors they witnessed there. They described how they grabbed family members and escaped from their homes in a hail of bullets. Whoever and whatever were left behind were burned.

Cradling his five children in the red dust outside the UNHCR tent that is now all he has, Kuide Pehe Ferdinand described the chaos when the attack began. "I had too many children to save when the rebels hit. We tried to pick them all up, but one of my baby girls is disabled and we had to leave her. When I went back, they had burned the house with my baby inside."

The Audgines were also grieving for a loved one killed after the rebels set fire to their home. "I can't even eat, I feel such sadness now," said Rosaline, mother of nine, whose elderly father was burned alive. She said she could do nothing to help him, as he shouted to them from within the flames. She and her children are a few of the many people in the camp who have shaved their heads in a traditional gesture of mourning.

The International Red Cross recently reached Toulépleu, and said it found a town almost completely razed to the ground.

HRW has documented the executions of elderly people who were unable to escape rebel attacks. It says they were held captive in their villages by the pro-Ouattara rebels, and has evidence that more than 30 were executed. One 67-year-old woman from the village of Doké told HRW that pro-Ouattara fighters had taken several captives out each day – often men and women between 60 and 80 years old – and executed them at point-blank range.

The pro-Ouattara forces have denied killing civilians in their advance upon Abidjan, blaming any deaths on Gbagbo's soldiers. Those standing guard at the border crossing with Ivory Coast near Toe Town, eastern Liberia, were in victorious mood when interviewed by the Guardian. In their smart camouflage gear and with AK47s slung around their necks, they swaggered up to the barrier across the bridge between the two countries.

"I pray for democracy in Ivory Coast and that the will of the people will be respected," said "Angelou", their commander, gripping his gun. As he talked, the sound of gunfire cracked from the forest behind him and his troops. "We don't have problem with civilians. If you see someone's died, it's because he's taken up a gun. If he's taken up arms, he is not a civilian, he is my enemy."

The conflict threatens to cause a wider humanitarian crisis in the region. More than a million people have been internally displaced within Ivory Coast, while more than 125,000 have crossed the border into Liberia, a country that itself has been devastated by 14 years of civil war. Many Liberian communities are sheltering refugees, but barely have enough food for themselves, and there are fears the crisis will destabilise Liberia's fragile peace.