France’s newest prime minister is preparing to take office on Wednesday just as a nebulous online protest movement promises a day of strikes and to “block everything” in the country.

The day of protest, a rejection of proposed austerity measures fueled by anger against President Emmanuel Macron, has been expected for weeks. But it arrived at a moment of extreme political volatility, less than 48 hours after the government lost a confidence vote and collapsed, opening a sudden vacuum of power.

Mr. Macron moved swiftly to fill it. Late on Tuesday he appointed Sébastien Lecornu, formerly the defense minister, as the new prime minister, a job that he is supposed to begin around midday.

By naming a center-right ally, and a proven loyalist, Mr. Macron appeared to double down on an approach that has burned through four governments already, three years into his second term. The move was interpreted by the president’s critics as more of the same and was sure to inflame the antigovernment protest movement, which calls itself Bloquons Tout, or Let’s Block Everything.

Many fear it could build like the Yellow Vest movement, which started online in 2018 and led to months of chaotic and sometimes violent protests across the country, finally petering out after the government spent nearly $20 billion appeasing it.

Bruno Retailleau, the country’s outgoing interior minister, said 80,000 police officers would be deployed around the country, on guard for attempts to block essential infrastructure including airports, public transportation lines and stations, power plants and water treatment centers.