More than 16,000 people rallied in Armenia's capital on Wednesday to protest the election of former president Serzh Sarkisian as prime minister, viewed by the opposition as a power grab.

Protesters staged a day-long march through central Yerevan, briefly blocking entrances to government buildings, and in the evening gathered in the Republic Square where a 30-metre Armenian national flag was unrolled.

Some demonstrators banged drums and shouted: "Armenia without Serzh!"

Protest organisers have vowed to mount a nationwide campaign of "civil disobedience" in opposition to the Kremlin-backed Sarkisian who was on Tuesday elected to the post of prime minister after a decade serving as president.

Opposition MP Nikol Pashinyan - the leader of the pro-Western Civil Contract opposition party - has called for "a peaceful velvet revolution" and urged his supporters to set up "revolutionary committees" across the country.

But the number of demonstrators dwindled on Wednesday, compared with Tuesday's rally of some 40,000 - the largest Armenia has seen in years.