Tens of thousands of people held protests across Mexico, joining tearful families of 43 missing students demanding their return, amid fears a police-backed gang executed them.

Parents of the victims travelled from Guerrero, where the students disappeared, to head a march of thousands of people in Mexico City on Wednesday, tearfully holding up pictures of their sons, and signs reading "we want them back alive".

People watched from the sidewalk in tears, holding their fists up and chanting "you are not alone!"

Al Jazeera's Adam Raney, reporting from the Guerrero state capital of Chilpancingo, said thousands also joined in the protest "to show their frustration with the government for what had been days of inaction".

But he said that many of the protesters had also expressed fears of reprisal from the drug cartels, adding, "this is a state where drug traffickers have more power than the government sometimes".

The young students disappeared on September 26 after municipal police officers working with a gang shot at buses seized by the aspiring teachers in the Guerrero city of Iguala, and took several of them away in patrol cars.

A mass grave containing 28 unidentified bodies was discovered on the outskirts of Iguala last weekend, in the same location where two hitmen from the Guerreros Unidos gang confessed to executing 17 students.

But authorities said it would take at least two weeks to confirm the identities of the bodies.

Demand for justice

"We are sad but we will fight until the end," said a 19-year-old from the missing students' teacher training school who declined to give his name for fear of reprisals.

"We demand that the president doesn't just talk and send more forces to Guerrero."

Thousands more protested in the southern state of Chiapas, with masked members of the Zapatista rebel movement taking part, without weapons but signs reading "We share your rage".

The government of President Enrique Pena Nieto has faced US and UN calls to solve the disappearance and investigate why gang-linked police attacked the students on September 26.

Pena Nieto deployed hundreds of federal forces to take over security in Iguala on Monday and disarm the local police. About 30 investigators from the prosecutor's office are in Iguala now.

While authorities are still investigating the motive, the spotlight has turned on Iguala Mayor Jose Luis Abarca and his wife, Maria de Los Angeles Pineda Villa, president of the local family services agency.

Governor Angel Aguirre said one theory being explored was that the wife was unhappy the students were in town, but he warned that the allegation she sent in the police should be taken with caution.

Pineda Villa's two brothers were members of the Beltran Leyva drug cartel until their deaths in 2009.

Authorities want to question Abarca over his role in the disappearance, but he and his wife have apparently gone into hiding.

Source: Al Jazeera and agencies