CHILPANCINGO, Mexico (AFP) - Protesters torched several trucks and tossed firebombs at a southern Mexican state government building on Saturday after the authorities said gang hitmen confessed to slaughtering 43 students in a case that angered the nation.

More than 300 students, many wearing masks, descended on the Guerrero state building in Chilpancingo, threw rocks at its windows and burned around 10 vehicles, including trucks and a federal police vehicle.

"We are asking the same thing as usual. We want to see our comrades alive," a masked student told AFP as others chanted "they took them alive, we want them back alive". Protesters had torched part of the Guerrero state government building last month.

Attorney-General Jesus Murillo Karam said on Friday that three Guerreros Unidos gang members confessed to receiving the students from the the police, killing them and incinerating their bodies.

The case has repulsed Mexico since gang-linked police officers attacked busloads of students in the Guerrero city of Iguala on Sept 26, in a night of violence that left six people dead and the 43 missing.

The confessions may have brought a tragic end to the mystery, which has sparked international outrage and protests in the biggest crisis of President Enrique Pena Nieto's administration.

But parents of the missing and fellow students at their teacher-training college near Chilpancingo refuse to believe the authorities until they get DNA results from independent Argentine forensic experts.

"It appears that the federal government, with great irresponsibility, is interested in closing this matter because it's all based in testimony. There is nothing definitive," said Mr Meliton Ortega, uncle of a missing student.

In taped confessions, gang suspects said they bundled the students in the back of two trucks, took them to a landfill in the town of Cocula, killed them and used fuel, wood, tires and plastic to burn their bodies for 14 hours.

The students had traveled to Iguala to raise funds but hijacked four buses to return home, a common practice among the young men from a school known as a bastion of left-wing activism.

The authorities say the city's mayor, worried that they would interrupt a speech by his wife, ordered the police to confront them. The officers shot at several buses, leaving three students and three bystanders dead.

The authorities have arrested 74 people, including the ousted mayor Jose Luis Abarca, his wife Maria de los Angeles Pineda, 36 police officers and several Guerreros Unidos operatives.

If the confessions are true, the mass murder would rank among the worst massacres in a drug war that has killed more than 80,000 people and left 22,000 others missing since 2006.

The Iguala case has undermined Mr Pena Nieto's assurances that the authorities were finally reducing the cycle of murders plaguing the country.

A protest was planned in Mexico City on Saturday, one day after hundreds of people lit candles on the steps of the capital's Angel of Independence monument.

They spray-painted #YaMeCanseDelMiedo, which translates to #I'mTiredOfFear, on the attorney general's office.

On Twitter, #YaMeCanse became a trending topic after Mr Murillo Karam uttered the words at the end of his hour-long press conference on Friday.

"It's important to see how Mexican society will react, if it will remain apathetic as it has been for years," historian Lorenzo Meyer said. "If this doesn't shock us, nothing else can."

Mr Murillo Karam stopped short of declaring all the students dead, and said an Austrian university would help identify the remains.

But the chief prosecutor added that there was "a lot of evidence... that could indicate it was them". Parents of the missing say they will not accept they are dead until independent Argentine forensic experts deliver DNA results.

Last month, two hitmen had already confessed to killing 17 of the students and dumping them in a mass grave near Iguala. But officials later said none of the students were among the bodies.

Some parents said the announcement of the confessions was aimed at allowing Mr Pena Nieto to leave Sunday on a major trip to China and Australia, which has been shortened due to the crisis.

"They want Pena Nieto to go on this trip," said Mr Felipe de la Cruz, a spokesman for the families.