MOSCOW, JUNE 26 -- A series of sweeping changes in the Soviet economic structure -- from pricing to management and factory to ministry -- will start taking effect next January under a package of proposals endorsed by the Soviet leadership today.

Reform guidelines, approved by the Communist Party's policy-making Central Committee and released tonight by the official news agency Tass, unveiled a radical program that over the next five years would transform the Soviet Union from a heavilycentralized command economy to one run predominantly on contracts entered into by newly competing enterprises, exchanging goods through wholesale trade.

At a two-hour press conference tonight, leading Soviet economist Abel Aganbegyan said this week's Central Committee plenum marked a turning point on the path toward fundamental economic change, which has been championed by Soviet leader Mikhail Gorbachev in the face of entrenched opposition in some party and bureaucratic circles.

Aganbegyan said the country's leadership faced a choice: "Either we implement radical reform in management and free driving forces, or we follow an evolutionary line of slow evolution and gradual improvement. If we follow the second direction, . . . we will not achieve our goals."

The economist said the decisions made today by the 307-member Central Committee launched "a truly new period of restructuring, a period of cardinal breakthroughs."

Aganbegyan, a key adviser to Gorbachev, was considered a radical a few years ago for his critical analyses of the flaws of the Soviet economy.

The plenum adopted a package that included a much-debated law on state enterprises, and 11 draft decrees covering the reorganization of pricing, financing, technological research, gathering of economic statistics and other areas.

"It will all be done stage by stage," Aganbegyan said, but stressed that the law and the new decrees will be on the books by Jan. 1, 1988, to give economic managers a comprehensive picture of the direction of economic reform.

Aganbegyan today confirmed that the reforms eventually will lead to a rise in food prices, would allow -- in rare cases -- the closing of insolvent factories, and over time reduce the proportion of factory orders placed by the state to less than 30 percent.

The main system of economic distribution will handled through contracts, negotiated between the state, factories and collective farms. The latter two will be run on a "self-financing" basis, which will allow them to distribute their profits, after the deduction of fixed costs to the state.

Since Gorbachev came to power in March 1985, he has emphasized greater flexibility for local economic managers and launched limited experiments testing the concepts of self-financing and "cost-accounting."

In introducing his new program, endorsed by the full 11-member ruling Politburo, yesterday, Gorbachev declared the effects of the initial stage of reform to be "insignificant." This, he said, justified a more far-reaching overhaul.

The scope of the reforms ranges from a new financing system, which would offer credits to factories to be repaid with interest, to the election of economic managers on a competitive basis.

The draft document spells out a timetable for the reforms, setting 1991, or start of the 13th five-year plan, as a final deadline for implementation of the new management system. The proposed law on state enterprises, given a qualifed endorsement today by Aganbegyan and due to be adopted Monday by the Supreme Soviet, the nominal parliament, will lead the way, going into effect next year. A program to "normalize" the consumer market should be in effect by 1990, the documents said.

In the meantime, the Central Committee today "particularly" emphasized the need to abrogate all old laws that contradict the pending reforms, and stressed that "economic activity other than prohibited by law is permitted to enterprises," a break with long-held Soviet assumptions.

According to the documents released tonight, the Soviet Union in four to five years will effect a transition to the "marketing of capital goods by way of wholesale trade." The guidelines propose that only "particularly scarce" capital goods will continue to distributed through allocation.

The program envisages the creation of "territorial supply-and-sales agencies" and "manufacturers' chain stores" to replace the giant state supply agency, or Gosnab, that now controls the flow of supplies.

The supply-and-sale organizations will be "independent self-sustained enterprises," servicing small producers and users.

The pricing system will be reformed only after "the broadest discussion in the country, considering its political and social significance," the documents said.

Aganbegyan today called the current system of state-fixed prices "irrational."

He predicted that basic food prices, which have been fixed for decades, will rise -- but not without compensation and notice, and not before 1990.

The plenum documents said that the reforms also will lead to a rise in prices of raw materials and fuel.

By encouraging competition between factories, particularly over state contracts, the reforms are aimed at breaking up "the manufacturer's monopoly and diktat over the consumer," according to the documents.

The reforms still leave a pivotal role for the central planning agencies, particularly Gosplan, which will be relieved of day-to-day oversight of the economy and charged with developing five-year plans and longer range targets.

The guidelines also give a greater role to "territorial" organizations and local governments by giving them control over medium-sized and small enterprises, a greater share of state revenues and a greater role in supplying local populations with local goods.

Aganbegyan today emphasized that the reforms will not lead to unemployment in the Soviet Union, nor, he insisted, in other ways alter its commitment to socialism. But like Gorbachev, he stressed the role of the now-developing cooperative sector and the limited self-employment allowed under a law that went into effect May 1.

Anticipating job relocation caused by the reforms, the guidelines today called for a network of self-supporting job placement bureaus and vocational guidance centers.