The Guatemalan authorities say they are expelling members of a non-profit "abortion boat" docked on its shores.
Officials said they had lied when they applied for tourist visas and would not be allowed to work in Guatemala.
The Dutch group, Women on Waves, offers free abortion services to women in countries where the procedure is banned.
It takes women in the early stages of pregnancy out to international waters, where the abortion is performed.
Abortion is only allowed in Guatemala when the mother's life is at risk.
A spokeswoman for Women on Waves confirmed that they had been told to leave the country immediately, but she said its lawyers had appealed against the decision.
La Prensa Libre newspaper said the four crew members being expelled were US citizens.Image copyright AFP Image caption The "Abortion Ship" is at Marina Pez Vela on Guatemala's Pacific coast
It is not clear if other members of the group have also been ordered to leave the country.
The Army said on Thursday it had been instructed by President Jimmy Morales to act, and would defend "human life and the laws of our country" by preventing the group from carrying out abortions.
The Women on Waves' boat docked on the Pacific Ocean port of Quetzal, in the city of San Jose, on Wednesday.
The group says it had a legal permit to sail in Guatemalan waters and the boat was being illegally "detained" by the authorities.
The group says more than 60,000 illegal abortions are performed in Guatemala every year, and most of the women who put their lives at risk at the hands of untrained professionals are poor.
"We respect religious beliefs but this [abortion] is a fundamental right in a democracy," spokeswoman Leticia Zevich told La Hora Newspaper.
However, Guatemala's Catholic Church, other religious leaders and politicians protested against the presence of the boat.
In most Latin American countries, abortion is either illegal or only allowed to save the life of the woman.