Egypt's President Anwar Sadat tonight announced sweeping law and order measures following last month's food riots, which he blamed on the Soviet Union.

In a two-hour speech, he said Communists were responsible for the two days of rioting in which about 80 persons were killed and some 800 injured. He said the Soviet Union is opposed to his liberalization policy that would encourage Western investment in Egypt.

Calling the riots "very dangerous," Sadat banged the table and wiped his brow in a rare show of anger as he demanded: "What is this outburst of thefts, of burning private cars? Why? Are there not private cars in Moscow? They have Zims, Moskviches and others."

During the rioting that forced the government to rescind moves that would have raised food prices, mobs vented their anger on such symbols of affluence as big American and German cars.

Saying plotters were behind the disorders, Sadat declared, "I shall never forgive them for carrying out this criminal machination . . . The idea was to tell foreign investors, 'Do not invest your money in Egypt because there is no security there.'"

"I am demanding that my people wake up and strike at such attempts," he said. He announced that the strict measures he decreed tonight will be submitted to a national plebiscite - the first of its kind here - next Thursday.

The new law would ban forming secret or armed groups, demonstrating or encouraging demonstrations, and strikes aimed at disrupting the economy.

Meanwhile, U.N. Secretary General Kurt Waldheim said after talks with Foreign Minister Ismail Fahmi that Egypt is flexible on the timing of a new Geneva conference on the Middle East, but insists on participation by the Palestine Liberation Organization.

In Damascus, Presidents Hafez Assad of Syria and Elias Sarkis of Lebanon announced after six hours of talks that they have established a "unified position" to end civil strife in south Lebanon, despite Israel's protests over Syrian troop movements in the area.