Tens of thousands of protesters have flooded the streets of the Mexican capital demanding the safe return of 43 students who went missing over a month ago, with hopes that the arrest of key suspects would bring answers.

Demonstrators chanted: "They took them alive, we want them alive," holding a large banner with images of the 43 college students. Their disappearance has drawn international outrage and turned into a full-blown crisis for President Enrique Pena Nieto.

Former mayor of the southern city of Iguala, Jose Luis Abarca, and his wife, Maria de Los Angeles Pineda, were arrested on Tuesday in a gritty neighbourhood of Mexico City as the main suspects in the case.

Their capture raised hopes that they could offer tangible clues about the students' whereabouts almost six weeks after they were attacked by Iguala police officers linked to the Guerreros Unidos gang.

Prosecutors accuse the mayoral couple of plotting the attack with the gang over fears the students would disrupt a speech by Pineda, who was head of the local child protection agency.

The night of terror left six people dead and the 43 students missing.

Organisers said 120,000 people marched in the latest protest over the case while police counted 60,000. Parents of the missing, joined by protesters, said the arrest was not enough.

"We demand that the government present our sons alive," said one of the fathers, Felipe de la Cruz, at a rally in front of the historic National Palace.

Slim hope

The investigation has led authorities to a dozen mass graves containing 38 bodies, raising fears of a tragic end to the mystery.

Officials say preliminary DNA analyses have shown that at least 28 of them are not the students, but their relatives do not trust the government and want independent Argentine forensic experts to give the final word.

Despite the gruesome discoveries, parents of the missing and some officials harbour slim hope, with Guerrero's interim governor saying the abductors are possibly moving them around to elude the search.

Alejandro Hope, a security expert and former intelligence officer, said the most likely scenario is that the students were killed.

"Why would they hold 43 people for 40 days without asking for a ransom? It makes no sense," he told the AFP news agency.

Thousands of security forces backed by drones and boats have been combing towns, mountains and rivers around Iguala, 200km south of Mexico City.

Recent high-profile abductions have ended tragically.

Source: Al Jazeera and agencies