An overtime ban also came into force on October 1. University hospitals in Helsinki, Tampere and Oulu are among those affected by the ban and mass resignations.
"It's really sad that trained healthcare workers are forced to defend their salary goals with such drastic measures," said Tehy's chair Jaana Laitinen-Pesola. She added that the mass resignations would paralyse the entire healthcare system.
Laitinen-Pesola said the proposed resignations would particularly affect specialised healthcare. However another 2,000 basic healthcare and social sector workers said they would be prepared to resign as well, if need be.
According to employers, nurses who join the mass resignations could face several problems, for instance with work benefits.
The union is aiming for a 24 percent pay increase. National Conciliator Juhani Salonius met with both sides in the dispute on Monday. He is to meet with them again on Thursday.
The government is to discuss the matter on Tuesday. Minister of Health and Social Services Paula Risikko has appealed to both sides to reach an agreement before the situation threatens patients' safety.