MEXICO CITY — With protests and little pomp, Enrique Peña Nieto on Saturday began his six-year term as president of Mexico, promising big spending and sweeping changes to bring peace and prosperity to a country troubled by drug violence and uneven economic growth.

“This is Mexico’s moment,” Mr. Peña Nieto declared in his inaugural address before a gathering of domestic and foreign leaders at the national palace, including Vice President Joseph R. Biden Jr., seated in the front row, while demonstrators kept far from the scene vandalized buildings and clashed with riot police officers outside.

Mr. Peña Nieto, 46, a lawyer who had been governor of Mexico State, has pledged to work closely with the United States to strengthen security and economic ties, which he believes would bring Mexico closer to a middle-class society and reduce the kind of drug war violence that has left tens of thousands dead in the past several years.

He made no promise to dismantle the drug-trafficking organizations, a focus of his immediate predecessor, Felipe Calderón. Instead, he unveiled a sweeping 13-point agenda focused more on domestic goals for crime prevention that would revise the penal code to attack impunity, give more attention to victims of violence, lessen poverty and hunger, improve schooling and even build new passenger train lines and expand Internet access.