YEREVAN (Reuters) - Armenians began voting Sunday in a referendum on constitutional changes that supporters say will strengthen democracy in the ex-Soviet state, but opponents warn will only help keep ruling party leaders in power.

The changes, approved in parliament in October, would curb the role of the historically powerful president and give more authority to the prime minister and parliament.

Supporters of President Serzh Sarksyan say the changes are needed to prevent political instability. Opponents say they will let Sarksyan slip into an enhanced prime ministerial role after his presidential term ends in 2018. Sarksyan has regularly dismissed those accusations.

“We will have a political stagnation in the country, if we don’t support these changes,” Sarksyan told reporters earlier this week.

“The changes will make cooperation between different branches of government more effective ... and facilitate economic development and the protection of human rights,” he said.

Sarksyan cast his ballot at a polling station in central Yerevan on Sunday and left without making any comment.

Group of opposition parties and opposition activists have held rallies in the central Yerevan for days, protesting the proposed changes. One opposition leader, Raffi Hovannisian, tore his ballot at the polling station in protest.

“The system, which is based on one person and one party, who just want to strengthen their positions by this referendum, is unacceptable,” Hovannisian told reporters.

Under the changes, the president will no longer be elected by popular vote, but by parliament. The winner would stay in the job for seven years instead of the current five, but would only have largely ceremonial powers.

Most of the role’s current powers would pass to the prime minister and parliament.