BANGKOK (Reuters) - An international rights group on Tuesday called for the release of a Thai anti-junta activist in his tenth day of a hunger strike after he was detained for campaigning against a military-backed draft constitution.

Thai voters overwhelmingly approved a constitution backed by the ruling junta in an Aug. 7 referendum, paving the way for a general election in late 2017.

Ahead of the vote, the junta banned open discussion about the charter and criticism of the draft was made punishable by up to 10 years in jail. Critics of the junta say the constitution will enshrine the military's political role for years to come.

Despite the ban on campaigning, students were among the most vocal critics of the constitution.

On Aug. 6 Jatupat Boonpattararaksa, a student at Khon Kaen University in the country's northeast, was detained for handing out "vote no" leaflets.

He is being held at a prison in northeastern province of Chaiyaphum province and has been on a hunger strike since Aug. 6, said Prim Boonpattararaksa, his mother and lawyer.

Jatupat faces charges of violating the ban on campaigning.

The New York-based Human Rights Watch called for Jatupat's release on Tuesday and said he should be given access to medical treatment after he passed out on Monday.

"The junta should immediately free Jatupat and other activists who peacefully protested the proposed constitution," said Brad Adams, Asia director at Human Rights Watch.

"In the meantime, he should be under the supervision of doctors in case his health condition worsens."

Thailand has been ruled by a junta since the military took power in a May 2014 coup.

Junta spokesman Colonel Winthai Suvaree said Jatupat should seek bail.

"The court has allowed him to bail himself out so it is possible for him to leave the prison," he said.

(Reporting by Patpicha Tanakasempipat; Editing by Amy Sawitta Lefevre and Michael Perry)