Mr Sallah ran against President Yahya Jammeh, pictured, three years ago
A court in Gambia has charged an opposition leader with spying, sedition and holding an illegal assembly.
Halifa Sallah was arrested on Monday and remains in prison because he was unable to meet bail requirements.
The leader of the National Alliance for Democracy and Development (NADD) has been arrested several times before.
During elections three years ago he stood against Gambia's President Yahya Jammeh, who has been in power since a coup in 1994.
The charges against Mr Sallah said he had "obtained confidential information which could be directly or indirectly useful to an enemy", Reuters news agency reported.
He was also charged with holding an unlawful assembly "with intent to bring hatred or contempt or excite disaffection against the government", Reuters added.
Human rights groups say the president's security forces have used arbitrary arrest and other abuses to stifle political opposition and crack down on press freedom in Gambia - the smallest country on the African mainland.
A report by Amnesty International released in November said opponents of Mr Jammeh were subjected to daily rights violations, including torture and unlawful arrest.