In another post on Tuesday, he tried to rally younger voters, cautioning that a win by his opponent would lead to “a bottomless darkness.”

Still, the math does not appear to be in his favor.

Mr. Erdogan won 49.5 percent of the vote, versus 44.9 percent for Mr. Kilicdaroglu, according to the Turkish electoral authority. The third candidate, Mr. Ogan, received 5.2 percent, and his right-wing supporters seem more likely to opt for Mr. Erdogan in the runoff.

Going into the first round, most polls indicated a slight lead for Mr. Kilicdaroglu, but since the results came out, analysts have tried to explain why the opposition performed worse than expected and how it might rebound in the second round.

“It seems like they didn’t make a plan for the second round, the Plan B,” said Seren Selvin Korkmaz, executive director of IstanPol, an Istanbul-based research group. “And now the opposition will spent a couple of days to make that plan and also to clean the wreck of the disappointment of that night. This is the biggest mistake in this election.”

The six parties that backed Mr. Kilicdaroglu represent a disparate range of backgrounds and ideologies, including nationalists, staunch secularists and even Islamists who had defected from Mr. Erdogan’s Justice and Development Party.