Campaigning for next month's referendum on changing Chile's dictatorship-era constitution began on Wednesday, more than four months after an outbreak of unrest that triggered the ballot.

More than 14 million Chileans will be eligible to vote on April 26 on changing a charter established in 1980, under the military rule of Augusto Pinochet (1973-90).

It has been changed before by referendum, following a 1988 vote that paved the way for a return to democracy.

More than 30 people have been killed during protests, with Chile's security forces accused by some of a heavy handed response More than 30 people have been killed during protests, with Chile's security forces accused by some of a heavy handed response Photo: AFP / Martin BERNETTI

"So many tears and so much blood have been spilled so that we could have a different constitution," Camilo Sanchez, the president of the Young Communists, told AFP.

Demonstrations that have left more than 30 people dead first broke out on October 18 last year, initially against a modest metro fare hike in the capital Santiago.

But it quickly mushroomed into wider anger at social injustice.

One of demonstrators' main demands -- alongside cheaper education and health care -- has been a change to the constitution.

More than four months after protests began in Chile, they have shown little sign of abating despite a raft of government concessions More than four months after protests began in Chile, they have shown little sign of abating despite a raft of government concessions Photo: AFP / MARTIN BERNETTI

A month later, right-wing President Sebastian Pinera and his governing coalition agreed to hold the historic referendum.

"The government is committed to encouraging Chileans to participate and to ensure a fair, democratic and transparent vote," Pinera said on Monday following clashes between protesters and security forces at the weekend in the seaside resort of Vina del Mar.

Campaigning will end three days before the referendum.

Voters opting to change the constitution will be asked if the redrafting should be carried out by people specifically elected for the job, perhaps via the formation of a new constituent assembly.

The government favors a committee, half of which would be made up of existing lawmakers while directly elected members would form the other half.

Chileans would vote for the group in October. Members would then have nine months to come up with a new constitution, a period that could be extended by three months.

According to the last poll by Activa Research, 69 percent of Chileans were in favor of a change with only 14.5 percent against.