Marou Amadou has been arrested twice in a week
Niger's government has promised to investigate claims that opposition activist Marou Amadou was beaten after he was detained by security forces.
Justice minister Lompo Garba told the BBC the government was already looking into the claims and would keep the public informed.
Mr Amadou's colleagues said he needed treatment at a prison clinic before being taken for questioning.
He opposed President Mamadou Tandja's plan to extend his rule.
Reports from Niamey say Mr Amadou has now been charged with "creating an illegal association".
He had been charged earlier in the week with breaching state security, but judges released him from custody on Tuesday.
Hours later, he was detained again and fellow activists say they saw him through prison gates, and that he appeared to have sustained injuries.
Mr Garba told the BBC's Idy Baraou, in Niamey, that the government was "at work, conducting investigations on the allegations of torture against Marou Amadou".
"If necessary, the government will give further details to the public later," he said.
Members of his group - the United Front for the Protection of Democracy (Fusad) - said he had been taken to an unknown location by members of the Republican Guard.
Fusad is allied to opposition parties that have been campaigning against a new constitution that extends Mr Tandja's term by three years.
Niger's new constitution, which was approved in a referendum on 4 August, allows Mr Tandja to run in subsequent elections, potentially giving him the opportunity to stay in power for life.
The EU and France have criticised the referendum and called on Niger to restore a democratic framework.