As turmoil from the takeover spread from the streets to the markets, sources said the regional central bank had cancelled Niger's planned 30 billion CFA ($A76 million) bond issuance, scheduled for Monday in the West African regional debt market, following sanctions.
The African Union, the United Nations, the European Union and other powers have condemned the junta's overthrow of elected president Mohamed Bazoum last week - the seventh military takeover in less than three years in west and central Africa.
The coup has raised fears for the security of the surrounding Sahel region.
The United States, former colonial power France and other European countries have troops in Niger and had been working with the government battling militant forces.
International concern over the coup is also sharpened by Niger's position as the world's seventh-biggest producer of uranium, the radioactive metal widely used for nuclear energy and treating cancer.
Junta forces arrested the ousted government's mines minister, the head of the ruling party, and oil minister Mahamane Sani Mahamadou, who is also the son of former president Issoufou Mahamadou, the Nigerien Party for Democracy and Socialism (PNDS-Tarayya) said.
The interior minister, transport minister and a deputy had already been detained, it added.
The arrests confirm the "repressive and dictatorial" nature of the coup leaders, the party said in a statement, calling on citizens to come together to protect democracy.
The arrests were announced a day after Chadian President Mahamat Idriss Deby arrived in Niger to try to mediate between the coup leaders and the ousted government.
Late on Sunday he posted what appeared to be the first images of Bazoum since the takeover, showing him smiling and apparently unharmed.
Deby said he was trying to "to find a peaceful solution," without going into further detail.
West African regional bloc ECOWAS has imposed sanctions on Niger, including a halt in all financial transactions and a national assets freeze, and said it could authorise force to reinstate Bazoum, who was locked in his palace by members of his guard on Wednesday.
Niger, which is one of the world's poorest countries and largely depends on external aid and financing, was expected to issue two other bonds in the regional market on August 7 and August 17, on top of the cancelled July issuance, according to an issuance calendar of the regional debt management agency.
Later on Sunday, the junta accused France of also planning to launch an operation to free Bazoum.
The French Foreign Ministry did not confirm or deny the accusation but said it recognised Bazoum as the only legitimate authority in the country and was focused on protecting its own citizens and interests there.
The junta previously warned against foreign attempts to extract Bazoum, saying it would result in bloodshed and chaos.
The coup in Niger followed military takeovers in neighbouring Mali and Burkina Faso over the last two years, all of which have come amid a wave of anti-French sentiment.
Both countries have turned increasingly toward Russia as an ally.
On Sunday, supporters of the junta burned French flags and attacked the French embassy in Niger's capital Niamey, drawing tear gas from police.
The coup leaders, who have named General Abdourahamane Tiani, the former presidential guard chief, as head of state, said they overthrew Bazoum due to poor governance and discontent with the way he handled the Islamist threat.
Yevgeny Prigozhin, the head of Russia's Wagner mercenary group last week welcomed the coup in Niger, and said his forces were available to restore order.
The Kremlin said on Monday that the situation in Niger was "cause for serious concern" and called for a swift return to constitutional order.
Germany's foreign ministry said on Monday that the situation was still in flux, and that there was a possibility the coup could fail.
