(Reuters) - The Organization of American States (OAS) and opposition activists on Tuesday increased calls for Nicaraguan President Daniel Ortega to release all of the people they regard as political prisoners, following the death of a U.S. citizen in prison last week.

The Civic Alliance, which encompasses various opposition groups, announced a national strike starting on Thursday to put more pressure on the government. The opposition says more than 500 political prisoners remain in custody, while the government puts the figure at 132 people.

By Tuesday evening, Nicaragua’s four largest business groups had said they would participate in the strike.

“This will be a civil and peaceful strike that covers the entire national territory and all economic activities,” the business groups said in a statement.

The OAS’s permanent council met on Tuesday afternoon in Washington to discuss the crisis in the Central American country. In addition to urging the prisoners’ release, the OAS demanded “free, fair, transparent elections,” freedom of expression and the right to the peaceful demonstration, according to a resolution approved on Tuesday by the group.

Canada, which convened the OAS session, called for the “unconditional release of all prisoners before June 18, 2019, as agreed by the parties at the negotiating table,” according to the draft resolution.

On Monday, the government said it had begun the process of releasing 100 prisoners.

Tensions ratcheted up again in Nicaragua last week after a U.S. citizen who was arrested last year was killed in a shooting in prison.

Ortega, 73, is grappling with the worst political crisis since he returned to the presidency in 2007. Mass protests demanding his resignation last year sparked a crackdown by security forces. Some 320 people were killed and more than 600 imprisoned, according to the Inter-American Commission on Human Rights.