The UN rights office condemned Tuesday the "excessive use of force" by security officers in Colombia, after numerous deaths during days of protests against a proposed tax reform.

The Office of the United Nations High Commissioner for Human Rights voiced "profound shock" at the latest incident overnight in the city of Cali in which police are alleged to have "opened fire" on demonstrators, reportedly killing and injuring a number of people.

It called for calm on all sides ahead of a planned mass protest on Wednesday, and stressed that security forces should only use firearms as a last resort.

Spokeswoman Marta Hurtado told reporters in Geneva that the situation remained volatile.

"What we can say clearly is that we have received reports, and we have witnesses, (of) excessive use of force by security officers, shooting, live ammunition being used, beatings of demonstrators and as well detentions," she said.

Protesters in Colombia called Monday for a new mass rally after 19 people died and more than 800 were wounded in clashes during five days of demonstrations, according to official figures. 

Separately, a European Union spokesman said the bloc also condemned "the death of the people killed during those protests, reports are saying that amounts to at least 19 victims, and one police officer."

"It is really a priority to stop the escalation of this violence and to avoid any disproportionate use of force by security forces," spokesman Peter Stano said. 

"We trust the action of Colombian institutions to investigate and bring to justice those responsible for any abuses, violations of human rights and freedoms."

Measure of last resort

The UN rights office said that "given the extremely tense situation, with soldiers as well as police officers deployed to police the protest, we call for calm."

"We remind the state authorities of their responsibility to protect human rights, including the right to life and security of person, and to facilitate the exercise of the right to freedom of peaceful assembly," spokeswoman Hurtado said. 

"Law enforcement officers should abide by the principles of legality, precaution, necessity and proportionality when policing demonstrations. Firearms can only be used as a measure of last resort against an imminent threat to life or of serious injury."

Hurtado added that UN officials were trying to verify the casualty toll from the incident overnight Monday in Cali.

"Our office in Colombia is working to verify the exact number of casualties, and establish how this terrible incident came about in Cali," she said.

"We express our profound shock at the events there and stress our solidarity with those who have lost their lives, as well as the injured and their families."