MONROVIA, Liberia — George Weah, a former international soccer star, bounded onto the field in a stadium filled with tens of thousands of roaring fans on Monday. He had been there before. But this time, instead of playing a match, Mr. Weah was there to be sworn in as president of Liberia.

“Today we all wear the jersey of Liberia and the victory belongs to the people, to the peace and to our democracy,” said Mr. Weah, dressed for his inauguration in a long white caftan.

Mr. Weah was elected in a milestone race for Liberia. It was the first peaceful democratic transition of power in more than seven decades. It was also the first presidential election to face a legal challenge, which Mr. Weah overcame after months of wrangling.

On Monday in his first official address to the nation, Mr. Weah promised to prosecute corrupt government officials and ensure that Liberian businesses played a bigger role in the national economy.