YEREVAN (Reuters) - Tens of thousands of demonstrators rallied in the centre of the Armenian capital on Tuesday to protest as parliament voted to allow former president Serzh Sarksyan to become prime minister in the former Soviet republic.

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Sarksyan was president from 2008 and demonstrators said he was switching jobs but clinging to power. Under a revised constitution approved by referendum in 2015, the prime minister will hold power while the presidency becomes largely ceremonial.

Sarksyan’s ally Armen Sarkissian was sworn in as president last week after being elected by parliament and in March Sarksyan said he would become prime minister to allow him to share the benefit of his experience.

“I have enough influence and power to make the executive and legislative branches of power work effectively,” Sarksyan told parliament on Tuesday. Parliament voted 77 to 17 in favour of his appointment.

Armenia seceded from the Soviet Union in 1991 but remains dependent on Russia for aid and investment. Many Armenians accuse the government of corruption and mishandling an economy that has struggled to overcome the legacy of central planning.

The protesters massed in the main square and surrounded or blocked entrances to governments buildings and organised sit-ins.

Local media said protests were also underway in Armenia’s two other big cities - Gyumri and Vanadzor. Police warned demonstrators to disperse or face tear gas.

On Monday, dozens were injured in scuffles with police and around 80 protesters were arrested, police said.

Opposition activists also held rallies in recent weeks to protest Sarksyan’s campaign to become prime minister and thousands blocked the centre of the capital Yerevan.

“I am declaring a start of a peaceful, velvet revolution in Armenia,” opposition MP Nikol Pashinyan told protesters in central Yerevan, many of whom were young people with no affiliation to a political party.