CHILPANCINGO (AFP) - Protesters fuming over the disappearance of 43 college students set fire to a southern Mexico state congress on Wednesday in another day of angry demonstrations over the presumed massacre.

Some 500 masked students and radical teachers broke into the empty Guerrero state legislature, igniting fires in the library and the chamber where local lawmakers hold sessions.

Moments earlier, protesters torched the education department's audit office in another part of the state capital Chilpancingo.

Violent protests have erupted in Mexico since authorities said Friday that Guerrero gang hitmen confessed to murdering the students and incinerating their bodies after corrupt police handed them over in September.

The case has shaken President Enrique Pena Nieto's administration, turning attention away from his internationally acclaimed economic reforms and undermining his assurances that his security strategy to combat years of drug violence was bearing fruit.

Pena Nieto traveled to China for summits this week, brushing aside critics who said he should have stayed home to deal with the growing unrest.

Protesters set fire to the regional headquarters of his ruling Institutional Revolutionary Party (PRI) in Chilpancingo on Tuesday, one day after blocking access to Acapulco's airport for several hours.

The state's governor, Angel Aguirre, resigned last month after protesters burned part of the government palace over his handling of the case. He was replaced by an academic, Rogelio Ortega.

Jose Antonio Crespo, a political science professor at Mexico's Centre for Economic Research and Teaching, said the protests are affecting Pena Nieto's international image.

"It gives the sensation that he's not in control, that he doesn't know what to do," Crespo told AFP. "It's yet to be seen how the protests can be stopped."

The demonstrations will not topple the government, however, because they are led by small radical groups for now and Mexico has "sufficiently strong institutions", he said.

The students vanished on Sept 26 after police shot at their buses in the city of Iguala, killing six people, and delivered the 43 to the Guerreros Unidos drug gang, authorities say.

The young men, who are from a teacher college known for its radical leftwing activism, had traveled to Iguala to collect funds but also stole four buses to return home when they came under fire.

Prosecutors say the city's mayor ordered police to confront the students over fears they would disrupt a speech by his wife.

Gang suspects told investigators that they killed the students in a landfill, burned their remains in a 14-hour bonfire, crushed them, and dumped them in a river, said Attorney General Jesus Murillo Karam.

But officials stopped short of declaring the young aspiring teachers dead, saying DNA tests on remains were pending.

Parents of the students, who deeply distrust the government, refuse to believe they are dead and say they will only trust DNA results from independent Argentine forensic experts.

The families will head a "National Information Caravan" that will leave from the students' teacher college in Ayotzinapa, near Chilpancingo, on Thursday and head to Mexico's north.

"The goal is to tell people that we continue to demand that the government find them, that for us they are alive and the search must go one," a relative, Epifanio Alvarez, told AFP.

Interior Minister Miguel Angel Osorio Chong told parents Tuesday that authorities will "redouble" search efforts and that the investigation remains open.

At the request of the families, the government signed an agreement with the Inter-American Commission on Human Rights to assist in the investigation.

The regional rights body can suggest "criminal complaints or contest decisions" the government took during the investigation, said deputy foreign minister Juan Manuel Gomez Robledo.

The charred remains found in the town of Cocula, near Iguala, were due to be sent Wednesday to forensic specialists at Austria's University of Innsbruck.

The university has handled several high-profile cases, including the identification of victims of the 2004 South Asian tsunami and the children of Russia's last tsar.