MEXICO CITY (AFP) - Thousands marched in Mexico City on Thursday to mark six months since the disappearance of 43 college students, with their parents demanding the suspension of elections in their home state.

Relatives reject government conclusions that the students were slaughtered by a police-backed gang in a case that has become the biggest challenge of President Enrique Pena Nieto's administration.

A Reforma newspaper opinion poll showed that his approval rating remained at a low 39 percent.

"Pena Nieto: Guerrero demands justice, not elections," read one sign held by demonstrators, referring to the southern state where the students disappeared, as they marched down the capital's main boulevard.

Earlier, parents of the students delivered a letter to the National Electoral Commission demanding the suspension of June 7 elections in Guerrero and other states.

"These last six months have been torture, painful, an agony for us," Meliton Ortega, uncle of a missing student, said outside the institute, where hundreds protested.

The four-page letter says the violence and infiltration of gangs in politics make it impossible to hold elections. Instead, it says, popular town councils could be named by people or organisations of "high moral standards." Parents said the election institute's president promised to respond by April 4.

Authorities say police in the town of Iguala abducted the aspiring teachers on the night of September 26 and handed them over to the Guerreros Unidos drug gang, which killed them and incinerated their bodies.

Only one of the 43 students has been identified among the charred remains that were found in a landfill and a river in the town of Cocula, near Iguala.

The attorney general's office issued a statement Thursday defending the investigation, saying it has been "transparent, exhaustive and in accordance with the law" and that it will continue looking for more suspects.

The statement said 104 people have been arrested, including 64 Iguala and Cocula police officers and 40 civilians linked to the Guerreros Unidos.

Iguala mayor Jose Luis Abarca and his wife, Maria de los Angeles Pineda, have been arrested over charges they ordered police to confront the students, who had travelled to the town in hijacked buses to raise funds for their left-wing college.

Investigators believe Abarca feared the students would disrupt a speech by his wife, and that the gang confused them with members of a rival criminal group.

Human rights groups have criticized the investigation, saying it relied too much on witnesses instead of physical evidence to conclude that the students were killed.

"We express our concern over the recent attempts by the Mexican government to discredit and not take into account the recommendations and observations of international human rights organizations," Amnesty International and other rights groups said in a statement.

Experts from the Inter-American Commission on Human Rights said last week that the fate of the students remains unclear and that they want to interview soldiers based in Iguala about the case.