Opposition leaders met last weekend with coup leaders to announce the formation of a transitional national council. The group said an interim president would rule the country until presidential and parliamentary elections.
The council "will dissolve all institutions, including the national Parliament," a spokesman was quoted by CNN as saying.
Military leaders last weekend closed air and maritime traffic near the country as a national security measure.
The coup came weeks before scheduled presidential elections. Acting President Raimundo Pereira and Prime Minister Carlos Gomes, the leading candidate, were taken into custody last week.
Mark Toner, a spokesman for the U.S. State Department, condemned the attack.
"We urge all parties to put down their weapons, release government leaders immediately, and restore the legitimate civilian leadership," he said in a statement.
The Economic Community of West African States, of which Guinea-Bissau is a member, was quoted by CNN as saying it had "zero tolerance for power obtained by unconstitutional means."