Poland's communist authorities opened substantive talks with the independent labor union federation Solidarity today in an effort to establish a peaceful partnership and restore social peace in the country.

It was the first time the two sides had started negotiations in an atmosphere free from strikes or threat of strikes. Their outcome could have important implications for Poland's uneasy relations with the Soviet Union.

If they are successful, and help restore peace here, the Polish authorities will have grounds for arguing that negotiation is a better tactic than confrontation in dealing with Solidarity. This in turn would vindicate their stand during talks here earlier this week with the Kremlin's chief ideologist, Mikhail Suslov.

In a move that symbolized the government's conciliatory mood, the official Polish news agency took the extradordinary step today of printing man-in-the-street interviews on the subject of political prisoners in Poland. It was an unprecedented display of frankness about the touchy subject in a Soviet Bloc country.

The official news agency PAP quoted Poles as saying they thought there were as many as 50,000 political prisoners in the country.

Meanwhile, the Roman Catholic Church in Poland condemned the "lying" of the country's news media before the reforms that followed last summer's labor revolt. A Polish bishops' pastoral letter said that "untruth dominated the mass media, information was falsified and perverse commentaries were made. Everyone said the press is lying, the radio is lying, the television is lying, the school is lying and finally the lies turned against the liars. Let those who insult the truth remember this."

The letter renewed the church's attack on atheism and said that it had been "forced upon Poland."

The negotiations, which were divided into separate working groups, could mark a significant turning point in the government's attitude toward Solidarity. Previously talks have been held in response to a specific crisis or upsurge of labor unrest.

It is too early to predict whether the new talks will be productive. The first round of discussions in the working groups was largely taken by each side restating their positions for the benefit of television and the press.

When appointed premier in February, Gen. Wojciech Jaruzelski promised a policy of greater openness toward Solidarity in return for a suspension of strikes. But the strike moratorium collapsed in March when Solidarity activists were beaten up by plainclothes police officers in the northern town of Bydgoszcz.

Partly as a result of the Bydgoszcz crisis, the government is making a more determined effort to treat Solidarity as a partner rather than an opponent.

The government knows that the only hope of having economic reforms accepted by the population is through agreement with Solidarity. This explains why the authorities have put such emphasis on the union's participation in a program of economic recovery.

The working groups that opened talks today included one in Solidarity's international activity. The union's representative Bogdan Lis, rejected suggestions that visits by Solidarity leaders abroad were harmful to Poland's international position -- an obvious reference to the sensitive issue of relations with Moscow.

Another working group dealt with problems of law and order and allegations by Solidarity against the police.

In parliament, a government commission began work on creating a legal basis for the promised registration of an independent farmers' union known as Rural Solidarity.

Talks also began in two provincial towns, Radom and Suwalki, on local issues involving alleged abuse of official power and demands that Communist Party buildings be turned over to the health service.