WARSAW, Poland, Oct. 28 (UPI) -- Solidarity activist Tadeusz Mazowiecki, Poland's first non-Communist prime minister since World War II, died Monday at age 86, Polish media reported.
Circumstances surrounding his death were unavailable.
Mazowiecki was among the most respected moderate voices in Polish politics, pushing for the country's integration in the European Union, The Financial Times reported.
Mazowiecki became Poland's prime minister after the historic Round Table talks and elections in 1989 that brought an end to communist rule, and held the post until December 1990, Polska Agencja Prasowa reported.
Polish President Bronislaw Komorowski, in a statement on his website, praised Mazowiecki for the "courage to be wise" at critical moments in Poland's history.
Flags were flown at half-staff and Komorowski declared a period of national mourning.
Stephen Mull, the U.S. ambassador in Warsaw, tweeted, "On behalf of the United States, I express my sympathy after the death of Tadeusz Mazowiecki, a founder of democratic Poland."
"When it comes to prime ministers, he was the best premier we've had up to this day," Lech Walesa, the leader of the Solidarity labor union and Polish president from in the 1990s, told Poland's TVN television.
Walesa chose Mazowiecki to lead the non-communist government, selecting him from the group of advisers who negotiated with the communists to allow for partly free elections.
Walesa and Mazowiecki had a falling out in 1990 when Mazowiecki decided to seek the presidency. In recent years the two mended fences.
Mazowiecki, with the help of economist Leszek Balcerowicz, who became his finance minister, launched reforms that turned the country into a market economy, reversing the ills created by the Marxist economic system, The Financial Times said.
"He will have a permanent place in Polish history," Balcerowicz said Monday.
Mazowiecki was a member of a Catholic movement that tried to work with the communists in the 1950s and 1960s, serving several terms in the communist-era Parliament, The Financial Times said.
In 1980, he went to Gdansk to lead a group of intellectual advisers to help striking shipyard workers and build the legal and ideological structure of the Solidarity labor union.
After he was released from prison following the 1981 declaration of martial law, Mazowiecki pressed for negotiations with the communists.
After withdrawing from his challenge to Walesa, Mazowiecki became the U.N. special emissary to Bosnia and Herzegovina during the Balkans war and was elected to the Polish Parliament as a member of the center-right Freedom Union.