PARIS  The prosecutor at the International Criminal Court formally requested an arrest warrant on Monday for Sudan’s president, Omar Hassan al-Bashir, on charges of genocide, war crimes and crimes against humanity committed during the past five years of bloodshed in the Darfur region of his country.

The prosecutor’s pursuit of Mr. Bashir introduced new volatility to the already chaotic situation in Darfur. While some diplomats and analysts worried that the move would undermine efforts to negotiate peace and provide aid to the millions displaced by violence, others said it offered new leverage to pressure the Sudanese government to end the conflict in Darfur.

Bracing for reprisals, United Nations peacekeepers and aid workers stepped up security in Darfur and pulled out all but the most essential civilians. Sudan promised not to vent its outrage on them, but said it would unleash a “diplomatic war” to try to scuttle the case.

It was the first time the prosecutor of the International Criminal Court had brought genocide charges against anyone. It was also the first time the prosecutor had brought charges against a sitting head of state since the court opened its doors in 2002. Two other presidents, Slobodan Milosevic of Serbia and Charles Taylor of Liberia, were charged by other international war crimes courts, also while they were in office.