Thousands of protesters have marched in Mexico City following the murder of an environmental activist.

Samir Flores Soberanes, who was also a journalist, was shot twice in the head in his home in Amilcingo, south of Mexico City, on Wednesday.

The protesters held signs saying "Samir didn't die, the government killed him"; "Samir lives"; and "Justice for Samir".

As the march made its way through Mexico City, thousands gathered in Amilcingo to lay Flores to rest.

The reasons for the killing are not yet clear but a prosecutor has indicated it was linked to organised crime.

Flores was a longstanding opponent of the Proyecto Integral Morelos (PIM), a development project that includes two new thermoelectric plants and a 150km (93 mile) natural gas pipeline in the state.

His death came just days before Saturday and Sunday's public referendum on the plants.

The Peoples in Defense of Land and Water Front, of which Flores was a member, said he attended a public meeting about the project days before his death and challenged government representatives.

Mexican president Andres Manuel Lopez Obrador expressed regret about the killing but said the referendum would go ahead. "I'm very sorry about the murder," he said. "The consultation we have to continue because it is a process that was already agreed to."

Activists fear that the pipeline will contaminate the local water supply, which would predominantly affect the indigenous communities in the area.

Flores's family and friends were joined by many others in Amilcingo on Friday for a funeral procession through the streets. The procession paused at the offices of Amiltzinko Community Radio, where Flores worked as a producer.

The contents of a note found next to Flores's body were not disclosed by police. An environmental group he worked with said it believe the murder was linked to his opposition to the pipeline.

Officials said that they had no evidence that Flores' killing was linked to the project or the referendum. Morelos state prosecutor Uriel Carmona told Radio Fórmula that he believed the murder was connected to organised crime.

Environmental activists were killed in record numbers in 2017, the latest year for which there are figures. According to data gathered by Global Witness, 201 were murdered.

Mexico saw the second highest number of murders after Brazil, with 15 killings, a more than fivefold rise over the previous year.