Leaders of a dozen organizations that are part of a new leftist political movement calling itself Marcha Patriotica have been threatened by right-wing paramilitaries, one of the people targeted said Monday.

A letter from the Aguilas Negras (Black Eagles) militia declared the Patriotic March activists a "military objective" and accused the group of being a stalking horse for armed rebels, the head of the Fensuagro agricultural union, Alirio Garcia, said.

The right-wing faction also labeled the Patriotic March members as remnants of Union Patriotica, an above-ground political party launched by the FARC guerillas in the 1980s.

Thousands of UP members were systematically killed by the security forces and paramilitaries.

The Aguilas Negras, one of several new paramilitary groups to emerge after the 2003-2006 demobilization of the 31,000-strong AUC militia federation, told the threatened activists they only have "a few days left" to leave Bogota.

Marcha Patriotica burst onto the Colombian political scene last month with a massive peaceful march in the capital. The new organization and the established leftist PDA party represent the only real opposition to the conservative national-unity government of President Juan Manuel Santos.

Two Marcha Patriotica members have been slain and two others "disappeared" since the April event in Bogota.