Mexican forensic experts recovered 28 charred bodies from a clandestine grave on the outskirts of this city where police engaged in a deadly clash with student protesters a week ago, Guerrero state's chief prosecutor said Sunday.

State Prosecutor Inaky Blanco said the corpses were too badly damaged for immediate identification and he could not say whether any of the dead could be some of the 43 college students reported missing after the confrontation with police. He said genetic testing of the remains could take two weeks to two months.

Blanco said one of the people detained in the case had told investigators that 17 students were taken to the grave site and killed there. But he stressed that investigators had not confirmed the person's story.

State police and prosecutors have been investigating the Iguala city police for misconduct during a series of violent incidents last weekend that resulted in six shooting deaths and more than two dozen people injured. Investigators said video showed police taking away an undetermined number of student protesters after a confrontation.

Mexico mass grave

Ayotzinapa Teacher Training College students hold lit sticks during a demonstration in the streets of downtown Chilpancingo, in the Mexican state of Guerrero, following violent standoffs with police that ended last week. (Jorge Dan Lopez/Reuters)

Authorities have presented charges against 29 people in the case, including 22 police officers detained soon after the violence. Three of the suspects are fugitives, including Iguala's police chief. Blanco said they are still investigating the motive for the crime, adding that some of the police have connections to a local drug cartel.

Blanco said Saturday that some of those arrested had provided key clues that led investigators to the unmarked burial pits on an isolated hillside on the edge of Iguala, which is about 200 kilometres south of Mexico City. Speaking at a televised news conference Sunday, he said the site is in rugged terrain about two kilometres from the nearest road.

'We want them returned alive'

The prosecutor said the bodies had been put in the pits on top of branches and tree trunks, which were doused with a flammable substance such as gasoline and set on fire.

Vidulfo Rosales, a lawyer helping families of the missing students, said relatives of 37 of the young people already had provided DNA samples that will be used to determine if the recovered remains belong to any of the students.

As investigators worked at the grave site, up to 2,000 protesters blocked a main highway in the state capital of Chilpancingo demanding justice. "You took them alive, we want them returned alive," read a huge banner hung across the road linking Mexico City and Acapulco.

Jesus Lopez, an Acapulco street vendor whose 19-year-old son Giovani is among the missing, said he hoped the remains weren't those of the students.

Other relatives "told us that [the remains] were burned, and that they couldn't be the kids," Lopez said. "But we're really nervous."

Student protest at governor's home

Mexico's National Human Rights Commission opened its own investigation into the case for possible "serious human rights abuses," such as extrajudicial executions and forced disappearances by Iguala city police.

Mexico mass grave

Police officers in Guerrero state have been searching for 43 students missing since violent clashes last week that left six people dead. (Jorge Dan Lopez/Reuters)

The commission said in a statement Sunday that it had warned about the "delicate" situation in Guerrero, a southern state where poverty feeds social unrest and drug gangs clash over territory.

Anger over the discovery of the graves exploded Saturday night when a group of young people from the Aytozinapa teachers college attended by the missing protested outside the governor's residence in Chilpancingo. They threw Molotov cocktails and overturned a car after state officials told them they would not be allowed to travel to the graves to determine if the bodies are those of their missing classmates.

Like many other schools in Mexico's "rural teachers college" system, Aytozinapa is known for militant and radical protests.