Malawi police on Thursday announced the launch of an inquiry into allegations by rights groups that its officers raped and tortured women during demonstrations over presidential election results.

The usually peaceful southern African country has been gripped by a wave of protests since President Peter Mutharika secured a second term in May.

Riots broke out last week in Msundwe - a trading outpost west of the capital Lilongwe - when opposition supporters blocked a pro-government group from attending a public meeting.

One policeman was stoned to death during the unrest.

Rights group Gender Coordination Network (NGO-GCN) said Thursday that some police officers took advantage of the chaos to sexually assault women in and around Msundwe on the following day.

They urged authorities to "ensure thorough investigation into the following alleged rape, defilement and torture of innocent women and girls".

Malawi police have since set up a commission of inquiry to investigate the allegations.

"Different professionals... will investigate the matter in a transparent and independent manner," said police spokesman James Kadadzera in a statement.

"All suspects identified will be treated according to the laws of the land without favour."

The assaults allegedly took place on October 9 by police officers dispatched to quell the unrest, according to the NGO-GCN.

"Police went to these places on duty because they were in uniform and they used a police car," said NGO-GCN head Barbara Banda, adding that the officers "threw teargas in every direction".

"In one instance, the parents of one of the victims was asked to go into another room and the girl was raped."

Banda told AFP that while three cases of sexual assault had been recorded so far, more could be "unveiled" by the investigation.

Protest organisers Malawi's Human Rights Defenders Coalition (HRDC), also called for the allegations to be investigated.

"It is shocking that we have the police, who are supposed to protect people, abusing and victimizing women," said HRDC official Gift Trapence.

Protests have flared in Malawi since Mutharika narrowly won May's presidential election amid widespread allegations of fraud.

Opposition supporters are demanding the resignation of the electoral commission chairwoman, who they accuse of rigging the vote.

Most demonstrations have turned into violent clashes between groups, with security forces firing tear gas and water cannon to disperse the crowds.