LUBUMBASHI, Democratic Republic of Congo — As hundreds of police officers ringed the courthouse, the mob came barreling up the street. From blocks away, you could hear its hungry roar.

“I am Moïse!” the people yelled. “We are ready to die today!”

At that moment, Moïse Katumbi, a popular opposition politician who is the gravest threat to President Joseph Kabila’s rule, stepped out of a Mercedes van, wearing a white shirt, white pants and white shoes, the flag of the Democratic Republic of Congo knotted jauntily around his neck.

The crowd exploded in a celebratory frenzy. The police officers surged, swinging clubs, slamming protesters to the ground and firing tear gas. Clouds of acrid purple smoke cut through the tropical air.

Congo’s government, which is continuously lurching from one crisis to the next, is now playing a very dangerous game. Mr. Kabila, who faces term limits, is resisting international calls and rising pressure in Congo to relinquish power by the end of this year, as Congo’s Constitution requires.