KINSHASA, Democratic Republic of Congo — America’s top diplomat on Wednesday was hoping to showcase the potential for democracy and climate action here in one of Africa’s largest countries. But backsliding reality got in the way.

Within hours of Secretary of State Antony J. Blinken’s arrival in the Democratic Republic of Congo — as he repeatedly urged free, fair and “on time” presidential elections next year — its government arrested a key opposition leader.

Forced to depart from his talking points, Blinken acknowledged what he called a “setback,” noting that he was “concerned for any steps taken that could actually reduce political space.” Blinken said he would raise the matter with the Congolese government and ask for an explanation.

The government of President Félix Tshisekedi arrested Jean-Marc Kabund, who has been under investigation for charges that have not been disclosed to the public. Kabund, a lawmaker and former vice president of parliament, was a close ally of Tshisekedi until the two had a bitter falling out earlier this year.

Kabund then formed an opposition political party and called Tshisekedi, who is running for reelection, a “danger.” Kabund’s lawyer was quoted in local media saying his client had been accused of contempt for the comments.

The incident illustrates how fragile efforts remain to build democratic institutions in parts of Africa, undermining stability and furthering violence.

Blinken spoke after meeting with Congolese computer students who are studying election transparency and how to counter disinformation, a program partially paid for by the U.S.

Tshisekedi’s assumption of the presidency in January 2019 marked the first peaceful transfer of power in Congo since it gained independence more than 60 years ago, but it came as part of a power-sharing arrangement that left his legitimacy questioned. Since then, his administration has been marred by what the State Department has called “pervasive” corruption, as well as human rights abuses and efforts to silence critics.