Opposition politicians have rejected the date in opposition to the rulers installed by General Mohamed Ould Abdel Aziz after he ousted Sidi Mohamed Ould Cheikh Abdallahi, the country's first democratically elected president.
They had hoped that Gaddafi would broker a compromise between the two sides.
"We had been expecting mediation rather than support for one of the two parties," said Mohamed Ould Maouloud, a leader of the National Front for the Defence of Democracy [FNDD], an anti-government coalition.
"It shocked us that he [Gaddafi] adopted the date of June 6 for a return to legality, and we do not agree with the timetable set by the junta."
Gaddahi had earlier said in the speech: "Mauritania's political, economic and social situation is very fragile and cannot support an overload.
"If one must judge people for coups d'etats, then not just Ould Abdel Aziz" must be judged, but also leaders of all past coups in the west African country.
Gaddafi had held meetings with Adbel Aziz and opposition leaders in Nouakchott, the capital, and met Abdallahi in Libya last week.
The FNDD had said that abandoning the June 6 date for elections would be a condition of dialogue.
They have called for Abdallahi to be returned to power, in place of the military.
Gaddafi's intervention is the highest-level mediation since the coup, which drew international condemnation and AU sanctions on members of the military-backed government.