Federal police with assault rifles and riot-shields advanced into Oaxaca on Sunday, pushing past barricades of burning tires and tree trunks in this normally picturesque tourist destination wracked by five months of protests and violence.

Officers in bulky black helmets entered the city from several sides, flanked by armored vehicles, trucks mounted with high-pressure water cannons and bulldozers, and with helicopters roaring overhead.

Some residents emerged from their homes to cheer and wave white flags as police clambered over burned-out cars and moved past hijacked tractor-trailers, buses and other debris used to block streets as they easily marched toward downtown.

In other parts of the city, however, a street battle unfolded. A line of police buses barreling down a major throughway had most of its windows shattered by protesters hurling rocks and massive chunks of concrete, and demonstrators tried their best to shove back police, who used shields, clubs and the blunt end of their weapons to fight back.

Police used water cannons to drive back protesters in several areas.

As police neared Oaxaca's leafy central plaza, protesters torched cars and barricades of wood, choking the area with smoke.

What began in late May as a teacher's strike in this colonial southern Mexican city of roughly 275,000 spiraled into a chaos as anarchists, students and Indian groups seized the central plaza and barricaded streets throughout the city to demand the ouster of Oaxaca state Gov. Ulises Ruiz. 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, resisted repeated calls to send federal forces to Oaxaca until Saturday, a day after gunfire killed a U.S. activist-journalist and two residents.

Protest leaders urged those at street barricades not to respond to advancing police with violence, though some demonstrators promised a street-by-street battle for control of Oaxaca. 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. "What do our people have? Most have just come to bring them flowers."

The Interior Department issued an ominous statement demanding that protesters give up their occupation of the city immediately, but officials said Sunday they still hoped negotiations could avoid further bloodshed.

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.

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

The government news agency Notimex reported that a vehicle transporting federal police to Oaxaca crashed Sunday, killing one officer and injuring 12 others. Federal officials said they could not confirm that report, but protesters cheered wildly as it circulated Oaxaca.

The protesters estimated that around 4,000 federal police had taken up positions around the edges of the city. There were no official reports, however, on how many officers were sent to Oaxaca, which is the capital of the state of the same name.

Demonstrators have occupied Oaxaca's central plaza since the start of the teacher's strike, setting up a sprawling tent city and covering historic buildings with graffiti. Surrounding streets were mainly deserted Sunday, and just one restaurant was open. A small group of residents and tourists sat eating and chatting quietly inside.

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 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."

"Brad's friends and family admired his brave support for the downtrodden and willingness to act tirelessly upon his convictions. We believe he died doing what he loved," it said.

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.