Guinea-Bissau's African Party for the Independence of Guinea and Cape Verde (PAIGC) says it will never recognize a power deal between a junta that seized control last week and a group of opposition parties.
PAIGC national spokesman Fernando Mendonca said talks between coup leaders and some political parties on returning power to civilians are simply a "farce."
Mendonca said the only way restore constitutional order is to return power to those elected by the people. He also demanded the immediate and unconditional release of Interim President Raimundo Pereira and leading presidential candidate Carlos Gomes Jr.
The junta has not given a reason for the detention of Pereira and Gomes, who were arrested by soldiers after the April 12 coup, but say the PAIGC officials will be investigated by the "proper authorities."
The International Committee of the Red Cross (ICRC) says a delegate has visited Pereira and Gomes twice since Saturday.
"At first sight the two detainees looked in good condition," said ICRC spokeswoman Marie-Servane Desjonquères on Thursday.
The coup took place as presidential candidates were set to begin campaigning for a run-off election that was to be held on April 29.
Gomes won the first round of voting and appeared to have a comfortable lead in the run-off against former president Kumba Yala, who has strong ties to the military.
Guinea-Bissau was electing a new president to replace President Malam Bacai Sanha, who died in January after a long illness. Pereira was appointed interim president until voters elected a new leader.
The international community has roundly condemned the coup and called for a return to civilian rule.
Since winning independence from Portugal in 1974, Guinea-Bissau has struggled through a dictatorship, four coups and the 2009 assassination of a president. It has also become a transit point for international drug traffickers.
