Senegalese police said on Friday that they had arrested General Bora Colley, the man who ran Gambia's prisons, where human rights groups say perceived opponents were tortured and in some cases died.
Colley was made the director of prisons by Gambia's former leader, Yahya Jammeh, who lost an election last month but refused to step down.
Jammeh fled into exile in Equatorial Guinea last week as a West African regional military force stood poised to remove him.
Senegal surrounds tiny Gambia on three sides, and it spearheaded the operation to install opposition figure Adama Barrow, the election winner. Its police reinforced border checks following Jammeh's departure.
"These checks led to, among other results, the arrest on January 25, 2017, of Gambian General Bora Colley by police at the border checkpoint in Mpack as he attempted to enter Guinea-Bissau," the police said in a statement.
Colley was later handed over to Senegalese military authorities, it said. Neither Colley nor any of his associates could be reached for comment.
Jammeh seized power in a 1994 coup and ruled Gambia for 22 years. His regime grew increasingly brutal and his election defeat, which he initially acknowledged before a dramatic reversal a week later, was celebrated across the country.
Colley served as commander of the military camp in Jammeh's home village of Kanilai. He was appointed director of Gambia's prisons in 2012.
Human Rights Watch accused Jammeh's government of forced disappearances, arbitrary detention, and the torture of journalists, human rights activists, political opponents and critics. Gay, lesbian and transgender people were also targeted.
Many of those abuses were committed in jails, including the notorious Mile 2 Central Prison in the capital, Banjul.
More than 90 opposition members were jailed following a wave of peaceful protests that began last April. Two died while in custody.
UN officials, who were allowed into the country for the first time in 2014, found that "torture is a consistent practice" and "avoiding arrest is a necessary preoccupation for Gambian citizens".