MOSCOW, DEC. 4 -- The Supreme Soviet approved today a package of constitutional changes strengthening President Mikhail Gorbachev's executive powers but retaining the post of prime minister that last month appeared to be marked for elimination when the Soviet leader proposed the plan.

Gorbachev told the standing legislature that he plans to use his new powers to prevent the Soviet Union from falling apart and to ensure the fair distribution of food supplies over the winter. In replies to questions from deputies, he also promised a crackdown on pornography and tougher measures to protect law and order.

The president said authorities are planning to import 100,000 tons of meat and 1 million tons of milk monthly to maintain food consumption at last year's levels. Overall, he predicted food imports worth $1.3 billion at the new commercial exchange rate between now and March.

On another front of the struggle for food supplies, Gorbachev promised to set aside 12 million acres of land to allow ordinary Soviet citizens to have private plots.

The constitutional changes must be ratified by the supreme state body, the Congress of People's Deputies, which meets on Dec. 17, but this is expected to be a formality. After a perfunctory debate, the Supreme Soviet gave Gorbachev the right to begin streamlining the government without waiting for final endorsement by the Congress.

Most of the changes were outlined by Gorbachev last month at a session in which many deputies complained about a paralysis of political power in the country. He responded by promising to take personal responsibility for running the government, a move that appeared aimed at undercutting his unpopular prime minister, Nikolai Ryzkhov.

Now that the details of the proposed changes have become clear, it appears that Ryzkhov could hang onto the post of prime minister, albeit with reduced powers, as head of a new "cabinet of ministers." Alternatively, he may be offered the new position of vice president under Gorbachev at the Congress.

Responsibility for coordinating major domestic policies and settling nationality conflicts has now been vested in a new Federation Council, made up of the president, vice president and leaders of the Soviet Union's 15 republics. Decisions will be taken by a two-thirds majority, a procedure criticized by some republics on the grounds that it could infringe on their sovereignty.

Gorbachev justified the introduction of majority voting in the council, which formerly was an advisory body, by arguing that a requirement that decisions be taken by consensus was likely to produce impasse. He cited the case of the ruling Communist Party Politburo under leader Leonid Brezhnev, which, he said, was frequently prevented from taking overdue action because of the opposition of a single member.

The Supreme Soviet voted overwhelmingly to endorse the changes, despite complaints from some deputies that they would further undercut the authority of the legislature. Gorbachev rejected the criticisms, arguing that the proposals to streamline the executive branch had originated there. He said details could be fine-tuned in committee.

The president's spokesman, Vitaly Ignatenko, told journalists today that Gorbachev was working on a shake-up that will follow the constitutional changes. He said "a suitable place" would be given to former interior minister Vadim Bakatin, who was dismissed on Sunday following a campaign to unseat him by a conservative faction in the Supreme Soviet.

One of the main criticisms of Bakatin by the hard-liners was that he allowed the governments of individual republics to assume responsibility for law and order, abandoning central control over the police. His replacement, Boris Pugo, said in a newspaper interview today that it was necessary to reverse "centrifugal tendencies" that were complicating the work of law enforcement bodies.

Gorbachev said groups of workers would be formed to crack down on the black market in food and scarce consumer goods. He said a survey of 50 state stores had shown that many employees were setting aside choice goods to sell at inflated prices to private customers. "When they close, they have a third shift," he complained.

In reply to a question about "an avalanche of violence, sex, and other slime" since relaxation of press censorship, Gorbachev agreed that it was "high time to crack down." He said that draft legislation would soon be presented to ban pornography.

"A couple of days ago, I saw on television a certain professor citing ancient Chinese art to illustrate and propagandize interesting sexual positions. This came as a great shock to our public. People are asking: 'Does the government have any power at all?' " said Gorbachev, wagging his finger at the newly appointed director of state television who was sitting on a government bench.