The junta in Guinea-Bissau has said it is setting up a transitional government along with some political parties as criticism of the coup continues.
Representatives of all of the defeated candidates in the first round of the presidential election have reportedly condemned the coup last Thursday.
Soldiers toppled the government over its alleged plans to reduce the size of the army.
No president has completed a term since independence from Portugal in 1974.
A delegation from the regional bloc Ecowas is hoping to persuade the junta to hand back power.
The latest coup happened just before the second round of presidential elections on 29 April, called after the death in January of President Malam Bacai Sanha following a long illness.
The front-runner, outgoing Prime Minister Carlos Gomes Junior, is being detained by the army along with the interim President, Raimundo Pereira.
Trade unions have called for a general strike in protest at the coup.
A spokesperson for opposition parties, Fernando Vaz, said that existing political institutions, including parliament, would be dissolved and replaced by a National Transitional Council.
Details - including the size, composition and the length of the council's mandate - were being hammered out at a meeting on Monday, he added.
Out of 35 opposition parties, 22 were reported to be taking part.
Representatives of the five presidential candidates defeated in the first round of voting were at the meeting although they had not been invited, Portuguese TV channel RTP Africa reports.
On Monday, these five candidates condemned the coup, AFP news agency reports.
"We firmly condemn the 12 April military uprising and demand the quick return of constitutional order, return of constitutional order," Kumba Yala, the Social Renewal Party leader, was quoted as saying.
Mr Yala is a former president who came second to Mr Gomes in last month's poll but said he would boycott the run-off, alleging fraud in the first round.
On Sunday, the junta closed all Guinea-Bissau's air and sea borders after Portugal announced it was sending two navy ships and a military plane for a possible evacuation of its nationals.
The Portuguese ships could play a role in a possible military intervention force, analysts say.
'No ambition for power'
At the weekend, the Community of Portuguese Speaking Countries (CPLP) proposed "taking the initiative" in such a force - with a mandate to be defined by the UN Security Council.
Jose Ramos-Horta, the outgoing president of fellow former Portuguese colony East Timor, has said the junta leaders have invited him to mediate in the wake of the coup.
On Friday, Ecowas said it would not tolerate the actions of the military.
Correspondents says questions remain over the organisers of the coup, who do not seem to include the army's top brass.
The junta say they acted to prevent foreign intervention and a plot to "wipe out" Guinea-Bissau's army.
About 200 Angolan officers have been in the country for the last year to help with training and reforms to the bloated army, which has long meddled in politics and is said by Western intelligence agencies to play a key part in trafficking drugs.
Guinea-Bissau has been destabilised by the booming illegal drugs trade from Latin America, which uses it as a staging post for the European market.