Six city workers died of asphyxiation and 28 people were injured Wednesday when protesters set fire to part of a municipal building in the opposition-governed highlands city of El Alto near Bolivia's capital.

Two men and two women died in a bathroom where they apparently fled and another man and woman died on a terrace, officials said. In all six cases, carbon monoxide poisoning was the cause, said Polonia Pinto, the director of Holandes Hospital, where the victims were taken.

The deaths come amid heightened political tension as Bolivians decide in a referendum Sunday whether to amend the constitution so President Evo Morales can run for a fourth consecutive term in 2019. Morales campaigned Wednesday in a wealthy southern neighborhood of Bolivia's capital La Paz, which lies beneath the teeming and poorer El Alto.

The fire was ignited by protesters following a march by residents demanding better schools and more teachers.

Union activists allied with Morales' governing party apparently joined the march initiated by parents of pupils and university students.

Mayor Soledad Chapeton, a member of the opposition, demanded to know why police were not present to prevent the violence.

"They irresponsibly did not show up on time," she told reporters between tears.

Chapeton blamed the fire on provocateurs allied with Morales' Movement Toward Socialism. She said the attackers removed incriminating documents from the building and set fire to them along with furniture.

Chapeton's predecessor, Edgar Patana, has been jailed while under investigation for corruption that allegedly included creating phantom jobs to siphon off municipal funds.