France's governing Socialists have suffered big losses in municipal elections, with the opposition UMP claiming victory and the far right celebrating further gains.
UMP leader Jean-Francois Cope hailed what he called a "blue wave" of support for his centre-right party.
The far-right National Front (FN) was heading for victory in 11 towns, partial results indicated.
The Socialists have been hit by growing discontent over the economy.
Prime Minister Jean-Marc Ayrault acknowledged the vote was "a defeat for the government and the [Socialist] majority".
"This message is clear... The president will draw conclusions, and he will do so in the interest of France,'' he added, in an apparent reference to a likely cabinet reshuffle.
It was unclear when a new government might be announced, or whether Mr Ayrault would keep his job.
French President Francois Hollande is due to meet Interior Minister Manuel Valls on Monday morning, and later Mr Ayrault.
Mr Hollande will also make an address, "probably televised", later on Monday, Agriculture Minister Stephane Le Foll told French radio.
The Socialists were said to have lost 155 towns of more than 9,000 inhabitants, Mr Valls said late on Sunday.
Turnout in Sunday's second-round vote was low, which was bad news for Mr Hollande's Socialists as it was their supporters who were not voting, BBC Paris correspondent Hugh Schofield reports.
Marine Le Pen's National Front won control of 11 towns of more than 9,000 inhabitants.
The party has also captured the 7th district of Marseille, France's second largest city. The district has a population of about 150,000, which makes it the party's biggest win.
The centre-right UMP was said to have captured a number of key cities, including Toulouse, Quimper, Limoges, Saint-Etienne, Reims, Roubaix and Tourcoing.
The Socialists retained control of Paris, with their candidate Anne Hidalgo due to become the capital's first female mayor.
"I am the first woman to be mayor of Paris," she said.
"I am aware of the challenge that represents... I will be the mayor of all Parisians, male and female, without exception."
Her victory against her UMP rival was seen as a consolation for the governing party on a night of setbacks.
Voters were choosing councillors and mayors in more than 36,000 municipalities. FN candidates had won through to the second round in some 200 places.
The FN is widely expected to do well in the European Parliament elections in May.