About 40,000 demonstrators have gathered in Armenia's capital as a week of protests against the country's new prime minister showed no sign of dissipating.

Protesters are calling for the resignation of Prime Minister Serzh Sargsyan, the former president who was appointed this week under a new system that gives the prime minister expanded powers and diminishes the presidency.

The crowd that amassed at a central square on Friday blocked the defense minister's car before police forcefully cleared the way.

The head of the Organization for Security and Cooperation in Europe's human rights bureau called on Armenian authorities to ensure the demonstrations proceed peacefully.

A spokesman for the prime minister, Hovhannes Nikoghosyan, says OSCE isn't in a position "to fully and adequately evaluate the situation" since its Yerevan office closed last year.