Mexican federal forces disarmed a southern city's entire police corps and took over security on Monday after officers were accused of colluding with a gang in violence that left 43 students missing.

The deployment in Iguala, 200 kilometers south of Mexico City, came after President Enrique Pena Nieto pledged justice in a case whose motive remains a mystery.

Authorities discovered a mass grave on a hill outside Iguala over the weekend that contained 28 bodies, raising fears about the fate of the students, who were last seen in the city more than a week ago.

Authorities said it would take at least two weeks to get results of DNA tests to identify the badly burned corpses.

The case could become one of Mexico's worst slaughters since the drug war intensified in 2006, with 100,000 people dead or missing to date, and the most horrific since Pena Nieto took office in December 2012.

Convoys of the government's new paramilitary police force and army trucks rumbled down the streets of Iguala.

Officials said more than 400 police officers and gendarmes were dispatched to patrol the streets. An unspecified number of soldiers were assigned to man checkpoints.

"Good afternoon, God bless you. The gendarmerie is here," said an officer waving at people from his truck window.

Carla Flora, a 40-year-old educator, welcomed the federal forces' arrival because "we were at the mercy of who knows who".

"We felt unsafe. Organized crime was inside the municipal police," she said.

Witnesses said several students from a teacher training college known as a hotbed of radical protests were whisked away in police vehicles on the night of Sept 26 after officers shot at buses the youngsters had commandeered to return home.

Prosecutors said the Guerreros Unidos drug gang participated in the night of violence that left six people dead, 25 wounded and 43 missing.

Two hit men confessed to killing 17 of the students, saying they were told by Iguala's public security director to head to the scene of the shootings. A gang leader told them to execute them, authorities said.

But nobody seems to know why the violence happened. Amnesty International called on Monday for an "exhaustive" investigation into the disappearances.

Attorney General Jesus Murillo Karam said more federal investigators would arrive in the city of 140,000 and that he would head there soon.

National Security Commissioner Monte Alejandro Rubido Garcia said Iguala's officers were being sent to a military base to undergo evaluations while investigators check whether their guns were used in crimes.

He did not say how many were disarmed but state prosecutors said last week that they had detained 22 officers after interrogating more than 140.

Some 30 people in total have been detained. The mayor and public security chief are on the run.

"This incident is without a doubt shocking, painful and unacceptable," Pena Nieto said in a televised address at the National Palace in Mexico City.

"Mexican society and the families of the youths who are regrettably missing rightly demand that the incidents be cleared up and that justice be served," he said.

But parents of the missing students accuse state authorities of lying.

They voiced doubts the students died, saying pictures of bodies from the mass grave don't match their physiques.

Armed vehicles are positioned outside the municipality in Iguala, Mexico, on Monday. Officials worked on Monday to determine if 28 bodies found in a mass grave are those of students who were detained by Iguala police.? Eduardo Verdugo / Associated Press