It is "crucial" that hundreds of witnesses and victims are protected in the trial of a senior army officer accused of crimes against humanity in the Democratic Republic of Congo, legal activists said on Thursday.

Colonel Bedi Mobuli Engangela is accused of leading brutal attacks against civilians in the town of Kalehe between 2005 and 2007, including murder, mass rape, kidnapping and sexual slavery.

More than 900 victims have given evidence of the attacks allegedly carried out by Engangela and his men in the town of Kalehe in South Kivu province.

The trial began on August 11 in Kalehe and is due to last until the end of the month.

It is "crucial to guarantee the security of all the people who have agreed to act as witnesses, braving the fear of reprisals," said Dominique Kamuandu, of Lawyers Without Borders, in a statement, adding that many of those who carried out the attacks are still at large.

"Some victims have already been intimidated during the pre-trial phase," he said.

Engangela, 40, is accused of carrying out crimes against humanity during his military career and after he deserted to lead a rebel militia. He faces at least 20 years in prison if found guilty. Any damages will be paid by the state, which is cited as having ultimate civil responsibility.

For Lawyers Without Borders, the trial of such a senior former officer is "symbolic" and sends "a strong signal in the fight against impunity".

The United Nations presented a list of five officers, including Engangela, to the DRC government in 2009, accusing them of "sexual violence, including mass rape and other severe human rights violations".

A report by the UN High Commissioner for Human Rights in July 2013 also charged Engangela war crimes.