Charlot Salwai wants to increase the numbers in cabinet from 13 to 17.
Our correspondent said the protestors are likely to be from all sectors of Vanuatu society.
Meanwhile, the Daily Post newspaper reported the opposition leader, Ishmael Kalsakau, saying he has filed a legal challenge to the prime minister's move, saying extra ministries are far too costly at this time for the country.
The paper has calculated that four more ministries would cost more than $US4 million dollars annually.
