President Francois Mitterrand moved today to shore up the authority of his squabbling left-wing coalition by defending his austerity program and describing as "seditious" recent antigovernment demonstrations by policemen.

The president's attempt to silence his critics in a live 30-minute television interview came after several weeks of open quarreling involving senior socialist and communist officials. The sense of political drift also has been exploited by right-wing opposition leaders who have called for a referendum on the government's economic policies.

Mitterrand, whose popularity has sunk in recent weeks to the lowest level ever recorded by a French president, said there is no alternative to the government's present economic strategy. He appeared to rule out the protectionist policies favored by left-wingers in the coalition by saying that France has to accept the need for international competition.

Left-wing Socialists and Communists are concerned that the austerity measures, which are designed to halve the present trade deficit of $12.5 billion by the end of this year, could cause a sharp rise in unemployment.

Aligning himself with other western leaders, Mitterrand said the state's budget had to be reduced and balance restored to foreign trade and social security expenditures. He said France could not continue consuming without saving. But, he added, all austerity measures must be accompanied by steps toward social justice and ending unemployment.

Asked about the results of the Williamsburg summit, Mitterrand said he had doubts about the usefulness of such meetings, "at least in their present form."

He said the positive element in the summit--the personal relations between heads of state and government of the seven biggest industrial countries--was seriously undermined by "this astonishing diplomacy of beating of drums and public auctions," which prevented serious negotiations from taking place.

Mitterrand hinted that France might withdraw from participating in such summits, including next year's conference in London, "unless of course the methods are radically changed," he said.

His criticism of the results of Williamsburg and of the continued rise in the price of the dollar appeared at least partly designed to meet concerns expressed by the Communists and left-wingers in his own Socialist Party. He has made similar veiled threats against participating at summit meetings in the past.

The president said he had ordered disciplinary measures against senior police officers accused of condoning demonstrations by about 4,000 policemen in Paris on Friday. Leaders of right-wing trade unions involved in the demonstrations were dismissed from their posts, and officers commanding police units that allegedly "fraternized" with the demonstrators were suspended from duty. Earlier, two of France's top policemen lost their jobs.

Criticizing what he called "these seditious events," Mitterrand said it was his duty to see that "the authority of the state is respected."

Mitterrand's disciplinary measures have been criticized by several police trade unions and have sparked concern that they could lead to fresh protests.

The president's broadcast was the first time he has directly addressed the nation since the steep decline in his personal popularity that began two months ago after regional elections. According to the latest polls, only 33 percent of Frenchmen say they are satisfied with Mitterrand and 50 percent say they are unsatisfied.

The popularity rating for Charles de Gaulle and Georges Pompidou during their terms as president never sank below 50 percent. The lowest level of support recorded for Mitterrand's predecessor, Valery Giscard d'Estaing, was 35 percent after six years in office.

Mitterrand's slump in popularity contrasts with continued high ratings for his finance minister, Jacques Delors, who has been largely associated with the implementation of the austerity program. The independent left-wing daily Liberation today attributed these contrasting ratings to a public perception of Mitterrand as hesitant and undecided on economic strategy.

Despite his public support for the Delors plan, Mitterrand has at times seemed torn by the arguments of left-wingers who have called for France to develop its industry behind protectionist trade barriers. He also is perceived as willing to make political concessions to avoid a rupture with the Communist Party.