At least 20 protesters taking part in recent demonstrations in Colombia appear to have been killed by police, Human Rights Watch (HRW) said in a report released on Wednesday.

The international monitoring group outlined other violent actions against protesters by police, including sexual abuse, beatings and arbitrary detentions.

HRW is investigating "credible reports" of 68 deaths and other incidents of violence during the civil unrest that started out on April 28 against a proposed tax hike.

So far, the rights organization has confirmed 34 deaths occurred in the context of the protests, including two police officers.

Colombian protester deaths

HRW said that "credible evidence" suggests that there were "16 cases in which the police appear to have killed unarmed protesters or bystanders with live ammunition."

In at least 15 of those cases, the victims had gunshot wounds in vital organs, the report found. Such injuries are consistent with an intent to kill, justice sector authorities told HRW.

The monitoring organization also documented cases of three people apparently killed by the impacts of teargas or flashbang cartridges.

A further protester died as a result of injuries after being beaten by police.

By comparison, Colombia's government has reported 18 deaths related to the protests and says an additional nine are under investigation.

The country's human rights ombudsman, meanwhile, reported late Monday that it had confirmed 58 deaths linked to the protests.

Incidents of gender-based violence

The HRW report documented two cases of sexual violence by police officers against protesters.

The report included Colombia's Ombudsperson's Office data, confirming two cases of rape, 14 cases of sexual assault and 71 other cases of gender-based violence by police officers, including slappings and verbal abuse during the protests.

Not 'isolated incidents'

"These brutal abuses are not isolated incidents by rogue officers, but rather the result of systemic shortcomings of the Colombian police," Jose Miguel Vivanco, HRW director for the Americas, said.

He called for "comprehensive reform that clearly separates the police from the military."

He said there was need for "adequate oversight and accountability" among the police.

President Ivan Duque has said all cases of police abuse will be investigated and duly punished. But Duque has insisted that these are isolated cases.

"Colombia is not a country that violates human rights, we have difficulties, but we face them with justice,'' presidential counselor for human rights, reported AP news agency, quoting Nancy Patricia Gutierrez on Tuesday.