WARSAW, DEC. 29 -- President Lech Walesa, in a clear signal that he wants Poland to remain the leading free-market reformer in Eastern Europe, nominated a young economist and entrepreneur today as his prime minister.

Jan Krzysztof Bielecki, 39, a member of the Polish parliament and a former underground activist in the Solidarity movement, heads a small political party composed of economic experts who have outlined a radical program for the swift sale of state-owned industry.

"The president's choice means that economic issues are most important," said Bielecki. "My candidacy has to be understood to mean that the future government will be composed of experts."

Bielecki promised to continue the free-market reforms begun last year by the Solidarity government of outgoing Prime Minister Tadeusz Mazowiecki. He indicated, however, that he wants the job only until parliamentary elections are held in Poland, probably in the spring.

"I consider this a service to the state and to Poland, and would like to end my service with free elections," said Bielecki, who had turned down the prime minister's job last week. Since then, he has had several meetings with Walesa.

The nomination of a prime minister is Walesa's first major decision since he took the president's oath a week ago. It follows more than two weeks of confusion, during which Walesa acknowledged he was having trouble finding anyone who wanted a premiership that would probably last for just a few months.

Walesa's press spokesman also announced today that Finance Minister Leszek Balcerowicz, the architect of Poland's "shock therapy" economic program, will be nominated to retain his position.

Walesa's economic advisers have said keeping Balcerowicz will reassure Western governments and investors that Poland plans to stick with the rigorous free-market reforms that have won it about $13 billion in Western credits. This is nearly double the credits pledged to any other post-Communist government in Eastern Europe.

Economic reforms here have forced a deep recession, putting more than 1.2 million Poles out of work and cutting workers' buying power by nearly one- third. But inflation has been halved, and the reforms have laid the foundation for rapid growth in private business.

The outgoing government, however, took a cautious approach to privatizing state-owned enterprises, which account for about 90 percent of the Polish economy. After one year, just five of the more than 7,000 enterprises had been sold for shares.

Bielecki, along with his technocrat colleagues in his Gdansk-based party, called the Liberal Democratic Congress, is expected to give the Ministry of Ownership Transformation a radical shakeup.

"We plan a privatization on a scale unknown in the world," Bielecki told a Polish newspaper this month.

"We are going to do shock-therapy privatization," Sen. Andrzej Machalski, one of Bielecki's closest colleagues in the Liberal Democratic Congress, said in a recent interview. Machalski said he expects to be named minister of industry.

"We have to privatize as quickly as possible and learn afterward from our mistakes," said Machalski, a businessman who heads the Polish Association of Employees. "I am afraid that there will be 2 or 3 million unemployed." He said Bielecki supports a program that would "commercialize" all state-owned industries "overnight." They would then be run on a for-profit basis by a "supervisory board" appointed by the government, he said.

"If the members of the supervisory board do a good job, they will be allowed to own a certain percentage of the stock of the enterprises," said Machalski. "We are of the opinion that changing ownership will increase production."

Perhaps the most intractable problem of Eastern Europe's transformation from socialism to capitalism has been how to find a fast, equitable way to transfer state-owned assets into private hands. The proposal apparently favored by Bielecki speeds up the process, but raises questions of equity. Critics say it will allow Walesa's government to name arbitrarily a new managerial class. They also say the government will be unable to find 20 or so "objective experts" to run each of the 7,000 companies. Bielecki has not had a chance to respond to these criticisms.

He is a graduate in economics from Gdansk University and speaks English well. When Communist authorities imposed martial law in 1981, he was fired from his job at an industrial training center and started a private timber-trucking business. He formed a consulting firm in 1985 and has helped advise the government on privatization in the past year.

In another signal that Walesa wants continuity with the outgoing government, it was announced today that Adm. Piotr Kolodziejczyk will stay on as defense minister.

Parliament is scheduled to consider Walesa's nominations on Friday.