In a press statement, Harmony co-leader Janis Urbanovics condemned the "secrecy and haste" with which the sale had been concluded and called for an emergency parliamentary session in an effort to prevent the sale going forward.
The sale of Citadele would leave the country "completely dependent on foreign banks" Urbanovics claimed.
The other Harmony co-leader, Riga mayor Nils Usakovs tweeted an invitation to other political parties - including the nationalist National Alliance that is in government - to back the call for a parliamentary debate, saying they should be "united against the theft of Citadele":
As reported by LSM, the government agreed on Tuesday to sell Citadele to a group of investors headed by US private equity firm Ripplewood Holdings.
However, full details of the deal and the sum involved will not be disclosed until a contract is signed some time before the end of the month, Prime Minister Laimdota Straujuma told the press.
September 30 is the deadline set by the European Commission for the state to offload Citadele, which was formed in 2010 from the wreckage of the notorious Parex bank after it was bailed out.