Updated

Federal police backed by armored vehicles and water cannons tore down barricades and stormed the once picturesque tourist city of Oaxaca, taking control of the city center that was held by leftist protesters for five months.

With helicopters clattering overhead, officers entered the city from several sides on Sunday, setting off fierce street battles. They marched up to a final metal barrier blocking the city 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.

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

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, even as police tore down the banners and tents that had served as their headquarters for months of often violent demonstrations.

Protest spokesman Roberto Garcia said 50 supporters had been arrested and police were searching houses, looking for protest leaders. Police did not immediately confirm that.

President Vicente Fox, who leaves office Dec. 1, had resisted repeated calls to send federal forces to Oaxaca until Saturday, a day after gunfire killed a U.S. activist-journalist and two residents.

The protests began in May as a teacher's strike in this colonial southern Mexican city of roughly 275,000. But the demonstrations quickly spiraled into chaos as anarchists, students and Indian groups seized the central plaza and barricaded streets throughout the city to demand the Ruiz's ouster.

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

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.

Once colonial and 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.

On Sunday, officers from the Federal Preventative Police climbed over burned-out vehicles, hijacked tractor-trailers, buses and sand bags blocking streets as they moved toward the plaza. Some residents emerged from their homes to cheer and wave white flags, 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.

Protesters said electricity was cut to the radio station being used to transmit information to demonstrators.

Protest leaders urged those at street barricades not to respond to advancing police with violence Sunday, though some demonstrators promised a street-by-street battle. Bertha Munoz, one of the movement's leaders, said that many demonstrators were peaceful.

"How can we confront them? We have already seen the R-15 (rifles) and AK-47s they carry," she said.

The Interior Department had said it hoped negotiations could end the Oaxaca standoff without further violence, but had no immediate comment as police moved to retake the city. Protesters said they had tried to contact the Interior Department late Sunday to negotiate, but were unable to contact anyone.

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!"

The protesters estimated that around 4,000 federal police had taken up positions around the edges of Oaxaca. There were no official reports, however, on how many officers were deployed.

Late Saturday, protesters gathered to mourn Bradley Roland Will, 36, of New York, who was killed during a shootout between protesters and men they claim were local officials in Santa Lucia del Camino on Oaxaca's rough outskirts.

Will, whose body was laid out in a white shirt and a glass-topped coffin at a funeral parlor near the square, was remembered as a video and documentary-maker devoted to the protesters' cause.

A video posted by Indymedia.org showed the last minutes of footage Will shot Friday, apparently including the moment he was hit by gunfire.

In a statement, Will's family said it was "grieving over the tragic and senseless loss of Brad's life."

U.S. Ambassador Tony Garza said in a statement that those who shot Will may have been Oaxaca police, and Mayor Manuel Martinez Feria of Santa Lucia del Camino said five men seen brandishing pistols at the time of the shooting had been turned over to authorities. He identified them as two members of Santa Lucia's city council, two of that town's police officers and a former justice of the peace from another town.

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