BANGKOK — A call for new elections by Prime Minister Yingluck Shinawatra of Thailand on Monday failed to quell antigovernment demonstrations, as tens of thousands of protesters massed outside her office and vowed to expel her powerful family from the country.

Ms. Yingluck’s announcement that she would “let the people decide the direction of the country” set in motion the dissolution of Parliament and the official endorsement of elections by the king. A royal decree set the election for Feb. 2, more than two years before the government was expected to finish its term.

Yet leaders of antigovernment demonstrations, which have left five people dead and several hundred injured over the past two weeks, vowed to press on with their quixotic campaign to rid the country of the influence of Ms. Yingluck and her brother, Thaksin Shinawatra, the billionaire tycoon and former prime minister whose policies have cemented the loyalty of voters in the most populous regions of the country.

A prominent Thai historian warned that continuing the protests despite the announcement of new elections could create “indefinite” conflict. The police said that well over 100,000 protesters filled the streets on Monday.