Guinean opposition leaders called Sunday for an independent investigation into police violence at recent demonstrations in which at least six people were killed.

"We learned with great dismay and indignation of the barbaric acts... perpetrated on the night of August 3 to 4" in the small town of Zogota in southeastern Guinea, two opposition coalitions said in a statement.

The statement by the two groups, Alliance for Democracy and Collective of Political Parties for the Finalisation of the Electoral Process, came after police shot at demonstrators in two different cities.

In Zogota, security forces opened fire on a protest against Brazilian mining giant Vale's employment of outside ethnic groups, leaving five dead.

On Tuesday, at least one person was shot to death and five others injured in the northeastern town of Siguiri, where local merchants were demonstrating against crime in their town.

"We strongly condemn these killings and demand the establishment of an independent investigation commission to determine responsibility," the opposition said Sunday.

The leaders also called for the "immediate release without conditions of those arrested (in Zogota and Siguiri) and detained in undisclosed locations."

They added that "these acts are a flagrant violation of the constitution and laws of the republic, as well as all the international human rights declarations that Guinea has signed."

On Thursday, the Guinean Organisation for the Defence of Human Rights (OGDH) also denounced the police violence, calling it "unworthy of a country that wants to be democratic".