Oaxaca, Mexico ? Federal police backed by armored vehicles and water cannons tore down barricades and stormed embattled Oaxaca on Sunday, seizing control of the city center from protesters who had held it for five months.

A 15-year-old boy manning one barricade was killed by a tear gas canister, human rights worker Jesica Sanchez said.

With helicopters clattering overhead, police earlier entered the city – once one of Mexico’s most popular tourist destinations – from several sides. They marched up to a final metal barrier blocking the center, but pulled back as protesters armed with sticks attacked them from behind, hurling burning tires. The air filled with black smoke and tear gas.

Some demonstrators used syringes to pierce their arms and legs, then paint signs in their own blood decrying the police.

As night fell, however, protesters decided to abandon the center and regroup at a local university. They pledged to continue their battle to get Gov. Ulises Ruiz to resign.

What began more than five months ago as a teacher’s strike in this colonial southern Mexican city of roughly 275,000 spiraled into chaos as anarchists, students and Indian groups seized the central plaza and barricaded streets throughout the city to demand the ouster of Ruiz, the Oaxaca state governor.

Protesters accused Ruiz of rigging his 2004 election and using thugs to kill or crush political opponents. They say his resignation is not negotiable and they won’t return home without it. The violence has driven tourists from one of Mexico’s most popular destinations, forcing hotels and restaurants to close their doors.

Mexican federal police push back protesters as they enter the city of Oaxaca. Federal police with assault rifles and riot shields advanced Sunday, pushing past barricades of burning tires and tree trunks in the normally picturesque tourist destination racked by five months of protests and violence.

Once breathtaking, Oaxaca’s main plaza is now covered with graffiti, having served as a home base for protesters who first seized the area in late May.

Police and state forces – often in plainclothes – have shot at protesters, setting off clashes in which at least eight people have died.

President Vicente Fox, who leaves office Dec. 1, had hoped negotiations could end the Oaxaca standoff without further violence and resisted repeated calls to send in federal forces until Saturday, a day after gunfire killed a U.S. activist-journalist, Bradley Roland Will, 36, of New York, and two residents.

On Sunday, officers from the Federal Preventative Police climbed over burned-out vehicles, hijacked tractor-trailers, buses and sandbags blocking streets as they moved toward the plaza.

Some residents emerged from their homes to cheer and wave white flags, while others fought to beat back their advance.

On one major street, police buses had most of their windows shattered by protesters hurling rocks and massive chunks of concrete.

The tense weekend standoff came after teachers agreed to return to work by today; their strike has kept 1.3 million children out of classes across the southern state. It was unclear whether the police presence would undermine that agreement.

In Mexico City, several hundred supporters of the Oaxaca protests converged on a hotel where Ruiz was rumored to be staying, damaging the grounds and screaming “Murderer! Murderer!”