The plight of the old and the homeless as Western Europe experiences one of its harshest winters in living memory was dramatized today by a devastating fire in a nursing home in France and a steadily mounting toll of people frozen to death.

Police said that at least 25 mostly bedridden elderly residents were killed when fire swept through a home in the village of Grandvilliers, north of Paris, early this morning. The cause of the blaze was traced to a frozen water pipe that triggered an electrical short circuit upon bursting.

Another 38 people, including many tramps who are accustomed to sleeping in the streets, are reported to have died as a result of the snowstorms and sub-zero temperatures that have held France in their grip for almost a week.

Across Western Europe, deaths attributed to the weather neared 100, and transportation in many countries was disrupted.

France's tragedies have coincided with an upsurge of concern here for the so-called "new poor" -- victims of rising unemployment and cuts in social security benefits. The poverty debate has assumed political overtones, with the right-wing opposition seeking to use the issue as a further means of discrediting the Socialist administration of President Francois Mitterrand.

To demonstrate his government's identification with the needy and deprived, Mitterrand today rushed to Grandvilliers, 30 miles north of here, with three of his ministers to comfort survivors of the fire. A regular weekly Cabinet meeting was delayed.

The bitterly cold weather -- with temperatures falling to as low as -12 F in some parts of France has encouraged public soul-searching about the social hardships of unemployment and homelessness. About 6 million people out of a population of 54 million are said to be living at or below the official poverty income of just above $5 a day.

During the past week, newspaper and television offices have been inundated with requests from middle-class Frenchmen asking how they can contribute to charities for the 22,000 people who are said to be homeless in Paris. Jolted into action by daily reports of people freezing to death on the streets of Paris, the city's public transportation system has invited tramps to spend the night in metro stations, with cots set up free of charge by the Army.

The sudden spurt of charitable activity prompted one radio commentator to remark caustically that "in France, solidarity begins at -10 C" -- which is 14 F.

The need for national solidarity in the face of an economic crisis that has hit France later than most other West European countries has been a recurring theme in speeches by Mitterrand during the past few weeks. He has described unemployment, which has reached the level of 2.4 million, as the greatest evil facing the country.

Growing unemployment and spreading poverty have become sensitive political issues for a left-wing administration that came to office in 1981 pledged to wage war on both. The number of people out of work has been growing sharply during the past two years at a time when the United States and some other West European countries began pulling out of recession.

Many of the "new poor" are drawn from the ranks of the new unemployed: people who have lost their jobs as the country attempts to modernize its aging industrial infrastructure. But the term also covers young people who have never had a job, immigrants, and single mothers unable to make ends meet on decreasing welfare payments.

The image of spreading poverty, which contrasts with the widespread notion of steadily increasing prosperity that took hold in France in the 1950s and '60s, has provided political ammunition for the right. Jacques Chirac, the neo-Gaullist mayor of Paris, recently claimed that about 600,000 unemployed were no longer entitled to full unemployment benefits.

The government has countered by increasing the level of the wealth tax and promising to use the money to fund emergency programs to help the poor.

One measure of changing attitudes toward poverty has been the comeback in the mass media of a 72-year-old priest who launched an "appeal for justice" for the homeless in 1954. After being largely ignored for the intervening period, the Abbe Pierre has suddenly reemerged as a public figure of the first rank whose views on poverty are solicited by journalists and politicians.

During the past few weeks, politicians including Prime Minister Laurent Fabius have met with the priest and pledged increased public support for his Catholic relief organization.