OAXACA, Mexico, Oct. 29 — Hundreds of federal riot police officers and soldiers took up positions outside this besieged tourist city in southern Mexico over the weekend, poised to end an increasingly violent protest that has shut the downtown for five months and left about a dozen people dead.

Riot police stood in formation across the main highway leading to Mexico City, facing down protesters waving white flags and protest signs 20 feet away.

A demonstrator, Gloria Juarez, approached the officers and stuck white roses and carnations in their black uniforms.

“It makes me sad that they want to kill my people,” she said. “I want to change their hearts.”

Later in the day, federal police moved in with water cannons and helicopters flew overhead.

Tension hung heavily in the air. Protesters appeared to be digging in at the barricades that they had constructed around town from sand bags, old tires, barbed wire and burned-out vehicles.

The federal government issued a statement ordering the protesters to “immediately hand over streets, plazas, public buildings and private property” so that officials could “guarantee public order and adherence to the law, as well as preserve respect for the population’s individual guarantees.”

But the protesters’ response was blunt.

“We’re going to resist the attack,” declared Florentino López, spokesman for the Oaxaca People’s Popular Assembly, or A.P.P.O., a loose coalition of interests that has laid siege to the city’s central square, or Zócalo, taken over radio stations and defaced much of the town with protest messages.

President Vicente Fox, in his final month in office, on Saturday ordered the federal troops in after three people, including a New York photographer, were killed in Oaxaca the evening before. The photographer, Bradley Roland Will, 36, who was shot twice in the abdomen, was a well-known activist on the Lower East Side who lived at times as a squatter in city-owned buildings.

The crisis in Oaxaca began in May when schoolteachers went on strike, an annual exercise that had usually led to a pay raise. But when Ulises Ruiz, the governor of Oaxaca State, ordered the protests broken up, leftists came to the teachers’ aid.

Soon, Oaxaca’s picturesque downtown was in the hands of the demonstrators, all with their own grievances against a local government they accuse of heavy-handed tactics.

“The roots of a revolution are gestating in Oaxaca,” says Zenén Bravo Castellanos, a teacher who heads the Revolutionary Popular Front, one group leading the protests. “The people of Oaxaca are tired of so many corrupt governments.”

After five months of standoff, with occasional bursts of violence, Mr. Fox’s patience apparently ended with the latest deaths. It was unclear, however, whether the federal troops intended to take over Oaxaca by force or to merely put pressure on the protesters to continue the months-long negotiations to end the impasse.

The teachers have reached a deal to return to classes, although some glitches remain. The other protesters, however, have vowed to continue to control the town until Governor Ruiz resigns.

Mr. Will, active in left-leaning political causes, was filming the standoff for an organization called Independent Media Center, or Indymedia. He was among the plaintiffs who had won a $120,000 settlement from New York City after officials demolished an East Fifth Street building despite a judge’s order that it remain standing until people squatting there could retrieve their belongings.

Mr. Will worked as a freelance photographer and reporter for the New York City chapter of the Indypendent, a radical collective that published a weekly newspaper and maintained a Web site dedicated to subjects like immigration, the Iraq war and the struggles of the developing world. In recent years he traveled widely in Latin America.

In Manhattan on Saturday night, about 100 people gathered outside the Mexican Consulate for a somber remembrance of Mr. Will. People lighted candles and incense and held a large banner that read, “Bring Brad Will’s Assassin to Justice.”

Witnesses in Oaxaca said a plainclothes police officer was responsible for his death. Protest leaders said such violence by the authorities would only inflame the already volatile situation.

The Oaxaca attorney general, Lizbeth Cana, labeled the protesters urban guerrillas and said it was understandable that local people were lashing out at them violently. “The people are fed up with permanent violence, threats and kidnappings,” she said, according to The Associated Press. But the mayor of a nearby town said the five men being detained for possible involvement in Mr. Will’s killing were not disgruntled ordinary local citizens but police officers and local officials.

“Oaxaca is a question mark,” Flavio Sosa, a leader of A.P.P.O., said before the federal forces arrived. “No one knows what’s going to happen.”

Antonio Betancourt and James C. McKinley Jr. contributed reporting from Oaxaca, and Colin Moynihan from New York.