A group of 26 Mozambique civil society organisations said on Monday $1.86 billion of government debt arranged by Credit Suisse and Russia's VTB Bank was illegal and should not be paid.
In a joint statement, the organisations said the loans breached a law requiring all borrowing and guarantees longer than a year to be approved by parliament. The debt also exceeded limits in the 2013 budget and were not entered into that year's budget, the groups said.
Paula Monjane from the Civil Society Budget Monitoring Forum, which coordinated the statement, said those who broke the law - on both sides of the transactions - should be investigated for corruption.
"The officials who broke the law in Mozambique need to be held accountable," she said.
"This includes the lenders and financial institutions which facilitated these loans. In defence of the common good and against the continued impoverishment of the Mozambican people, we do not want, do not accept and will not pay the debts."
Mozambique's parliament has agreed to set up a commission of inquiry into the issuing of the debt, which has pushed the foreign borrowing levels to 80 percent of the impoverished southern African country's GDP.
The controversial loans are an $850 million Eurobond issued in 2013 to build a tuna-fishing fleet, a $622 million loan to enhance maritime security and a $535 million loan to build shipyards for the offshore natural gas sector.
The tuna bond has already been restructured and the state company behind the $535 million loan missed a repayment of $178 million due on May 23, putting the government on track for default.
Financial watchdogs from Switzerland and Britain are looking into the arranging of the sovereign borrowing, which was done without consultation of the International Monetary Fund (IMF) or Mozambique's foreign donors.
IMF Managing Director Christine Lagarde said last month the lack of transparency behind the transactions was "clearly concealing corruption". She did not elaborate.
Credit Suisse has consistently declined to comment on the maritime security loan. VTB, which arranged the shipyard loan, says it expects the government to honour its sovereign guarantee.
Both banks say the tuna bond was above board and had the approval of the finance minister.