President-Elect Francois Mitterrand's proposal to take over huge chunks of French industry employing 600,000 people, as well as to nationalize all banks will probably be the most controversial issue in the French legislative elections that will probably be held next month.

The 11 big industrial groups concerned have a turnover of about 250 billion francs (about $45 billion), representing about a fifth of all French industry. Much of the takeover proposal is still vague.

According to Socialist experts, the present state sector represents 12 percent of French production, which would be brought up to 17 percent by the proposed new takeovers.

But unless Mitterrand can win support for what amounts to the third wave of nationalizations in 45 years, he is unlikely to obtain a sufficiently big parliamentary majority to carry out the changes. The present Parliament is dominated by the center-right coalition of Gaullists and Giscardists, with parties of the left holding only 200 of the 491 seats.

Except to cite Charles de Gaulle in favor of nationalizing "all forms of credit," Mitterrand has done little yet to defend the nationalization program, which would add to a public sector already employing a million workers representing 22 percent of the turnover of French industry.

The future right-wing opposition has already made it clear that it fears that this is only the beginning of a wave of new state ownership, particularly as Mitterrand has hinted at further takeovers once the first batch is approved by the National Assembly.

Historically, the left and right share the responsibility for the present state-owned sector, which began in 1936 with the Popular Front taking over the private railways. In the years following the liberation, de Gaulle ordered the takeover of the Credit Lyonnais, the Societe Generale and other banks, which formed a single banking entity.

More than 870 firms were bought out to form the national gas and electricity companies while 30 insurance organizations formed the basis of France's two dominant nationalized insurance firms. Renault, France's biggest manufacturing industry, was nationalized for collaborating with the Nazis.

Mitterrand's list is no less impressive. It includes Dassault, Rhone-Poulenc, ITT-France, Thomason-Brandt, CII Honeywell-Bull, CGE, Pechiney-Ugine-Kuhlman, Saint Gobain-Pont-a-Mousson and the steel giants Usinor and Sacilor.

There is an element of revenge in Mitterrand's proposal to take over the biggest French groups that have supported the outgoing administration and parts of multinational firms that the Socialists have said work against French interests. Most of the groups were formed in frantic takeover gambles in the last 10 years with open encouragement from the government.

The biggest as far as employes are concerned is CGE, with 170,000 staff. It dominates electronic equipment production and had a turnover of 42.5 billion francs (about $7.7 billion) last year.

Private trading banks have only a 20 percent share of the French system compared to 40 percent each for the nationalized companies and mutual credit firms like the Credit Agricole.

In referring to the "completion of the takeover of all forms of credit," the Socialists have never said what they intend to do with the major merchant banks like the Suez and Paribas. Foreign trading banks will be "controlled by the Bank of France," according to the Socialist platform.

So far the French business establishment has not intervened in the debate on nationalization to avoid any attitude that could compromise negotiations. But the private banks bought full-page advertisements in national newspapers during the campaign to plead for free enterprise. Their most vigorous support has come from the Gaullist Party.