PARIS — France vowed to combat terrorism with “a cry for freedom” in a giant rally for unity Sunday after a three-day spree of violence horrified the world. Police searched for a woman linked to the three al-Qaida-inspired attackers, but a Turkish official said she appears to have already slipped into Syria.

The rally Sunday is also a huge security challenge for a nation on alert for more violence, after 17 people and three gunmen were killed over three days of attacks on a satirical newspaper, a kosher supermarket and on police that horrified France and the world.

Hundreds of thousands of people marched Saturday in cities from Toulouse in the south to Rennes in the west to honor the victims, and Paris expects hundreds of thousands more at Sunday’s unity rally. More than 2,000 police are being deployed, in addition to thousands already guarding synagogues, mosques, schools and other sites around France.

The leaders of Britain, Germany, NATO and the Arab League are among dozens of world dignitaries expected to attend — as are French politicians from across the spectrum, and Muslim, Jewish and Christian leaders. Top European and U.S. security officials are also holding a special emergency meeting in Paris about fighting terrorism.

The rally “must show the power, the dignity of the French people who will be shouting out of love of freedom and tolerance,” Prime Minister Manuel Valls said Saturday.

“Journalists were killed because they defended freedom. Policemen were killed because they were protecting you. Jews were killed because they were Jewish,” he said. “The indignation must be absolute and total — not for three days only, but permanently.”

Al-Qaida’s branch in Yemen said it directed Wednesday’s attack against the publication Charlie Hebdo to avenge the honor of the Prophet Muhammad, a frequent target of the weekly’s satire.

French radio RTL released audio Saturday of Amedy Coulibaly, speaking by phone from the kosher supermarket where he killed four hostages, in which he lashes out over Western military campaigns against extremists in Syria and Mali. He describes Osama bin Laden as an inspiration.