The Organization of American States is urging Nicaraguan President Daniel Ortega to stick to his agreement to unconditionally free all political prisoners by the June 18 deadline.
At a Washington meeting Tuesday, an OAS resolution also demanded "free, fair and transparent elections" in Nicaragua along with freedom of expression and the right to peaceful protests.
The opposition plans to pressure the Ortega government by holding a nationwide general strike Thursday.
The government said Monday it already released more than 300 political prisoners. The opposition says about 500 still remain behind bars while officials put the figure at 132.
Nicaragua has been in political turmoil since a protests against social spending reforms last year turned into general anti-government protests, sparking a violent crackdown by security forces. Human rights officials say more than 320 people have been killed and about 2,000 wounded.
Ortega has refused demands to resign or hold early elections.