By TATIANA GUERRERO - Associated Press Writer

BOGOTA, COLOMBIA --The U.N. on Friday called for an investigation into the deaths of six organizers of a march protesting the Colombian government and paramilitary death squads.

The victims included union workers and human rights activists. They were killed around the time of the March 6 protest that drew tens of thousands of people.

"This office asks state authorities to guarantee prompt and efficient protection for those human rights defenders and the leaders of social organizations," the U.N. High Commissioner for Human Rights said in a statement.

Activists accuse President Alvaro Uribe's administration of lumping human rights workers together with rebels from the Revolutionary Armed Forces of Colombia, the country's largest leftist guerrilla group, supposedly giving the death squads a green light to attack the activists.

Many Colombians refused to join the March 6 protest, calling it a campaign to smear the government and the armed forces. President Alvaro Uribe's closest adviser, Jose Obdulio Gaviria, alleged the march was organized by the FARC.

A human rights organization called Movement of Victims of State Crimes said one paramilitary group sent a threat Wednesday by e-mail to organizations connected with the protest.

The paramilitary death squads were created by landowners and drug lords to battle leftist rebels. Some parts of the armed forces allied themselves with the death squads to fight what was regarded as a common enemy.

While tens of thousands of far-right paramilitaries have disarmed under a peace deal with the government, new groups have sprung up across Colombia.

The e-mail sent Wednesday said the group, called the Black Eagles, "would be implacable with those people who had organized the protest," said Ivan Cepeda, director of the human rights group.

Interior Minister Carlos Holguin rejected the threat and called the Black Eagles a "criminal organization."