Advocacy groups for immigrants expressed anger over Thursday's ruling, saying it ignored the rights of those affected and was based on bigotry against predominantly black Haitians.
"This is outrageous," said Ana Maria Belique, spokeswoman for a nonprofit group that has fought for the rights of children born in the Dominican Republic to migrants, such as herself. "It's an injustice based on prejudice and xenophobia."
The Constitutional Court's decision cannot be appealed, and it covers those born since 1929—a category that overwhelmingly includes Haitians brought in to work on farms and their descendants.
David Abraham, a law professor at the University of Miami, said the decision was part of a larger effort to keep Haitians from entering the Dominican Republic and to encourage self-deportation of those already here.
He cited the racial differences between the predominantly black Haitians and mixed-race Dominicans as well as Haiti's plight as one of the world's poorest countries.
"The fear of the Dominican Republic, of being pulled down to the level of Haiti economically and the 'blackening' of the country, has been an obsession of Dominican politicians for well over a century," he said.
Spanish-speaking Dominicans and Creole-speaking Haitians share the Caribbean island of Hispaniola and have a long history of troubles, including wars and massacres. Relations warmed after Haiti's devastating 2010 earthquake that killed an estimated 300,000 people, but tensions have since resumed.
The office of Haitian Prime Minister Laurent Lamothe declined to comment about the ruling.
Edwin Paraison, a former Haitian Cabinet minister who has been working to improve relations between the two nations, criticized the court and warned that the ruling could hurt Dominicans. "The sentence expresses a rejection of the Haitian diaspora while setting a dangerous precedent that can be reproduced, if appropriate action isn't taken, against other immigrant communities, including Dominicans, in several countries worldwide," he said in an email.
The Constitutional Court said officials are studying birth certificates of more than 16,000 people and noted that electoral authorities have refused to issue identity documents to 40,000 people of Haitian descent. It gave the electoral commission a year to produce a list of people to be excluded from citizenship.
The Economy Ministry recently calculated that about 500,000 people born in Haiti now live in the Dominican Republic, but it gave no estimate for the number of people of Haitian descent living in the country. The Dominican Republic's total population is a little over 10 million.
The debate over citizenship began to escalate in 2007, when electoral authorities refused to issue identity documents or return copies of them to Dominican-born people of Haitian descent. In 2008, several people challenged those decisions in court, including Belique, whose birth certificate was seized by government officials when she tried to enroll in a local university.
Until 2010, the Dominican Republic followed the principle of automatically bestowing citizenship to anyone born on its soil. But that year, the government approved a new constitution stating that citizenship will be granted only to those born on its soil to at least one parent of Dominican blood or whose foreign parents are legal residents.
Citing that constitution, the court ruled that all Haitian migrants who came to work in Dominican sugarcane fields after 1929 were "in transit," and thus their children were not automatically entitled to citizenship just because they were born here.
Dominican lawyer Cristobal Rodriguez said the court disregarded the principle of law retroactivity by applying the criteria of a new constitution approved in 2010 to people born decades earlier.
Rights groups and migrant activists said the decision would force many people underground and deprive them of basic needs and public services.
Activists said they would likely seek help from the Inter-American Commission on Human Rights, which in turn might submit the case to the Inter-American Court of Human Rights.
Jorge Duany, an anthropology professor at Florida International University who has studied the migration of Dominicans in the Caribbean, said the decision comes after countless years of friction between the two countries.
"The impact could be truly catastrophic," he said. "They are stigmatizing an entire Haitian population."
Associated Press writer Ezequiel Abiu Lopez reported this story in Santo Domingo and Danica Coto reported from San Juan, Puerto Rico. AP writers Trenton Daniel and Evens Sanon in Port-au-Prince, Haiti, contributed to this report.