The Soviet presence, symbolized by the Palace of Culture and Science, turned out to be very acute. Stalinism lasted in Poland until 1956—as in the USSR. The leader ("First Secretary") of the Polish Communist party, (PZPR), Bolesław Bierut, suddenly died in Moscow during the 20th Congress of CPSU in March, probably from a heart attack. By October, the new First Secretary, Władysław Gomułka, in a speech during a rally on the square in front of the PKiN supported the regime liberalization (so-called "thaw"). At first, Gomułka was very popular, because he also had been imprisoned in Stalinist prisons and as he had taken up the office of PZPR's leader, he promised a lot, but the popularity faded quickly. Gomułka was gradually tightening the regime. In January 1968, he forbade the performance of "Dziady", a classical drama by Mickiewicz, full of anti-Russian allusions. That was "the last drop of bitterness": then students went out on the Warsaw streets and gathered by the monument to Mickiewicz to protest against censorship. The demonstrations spread throughout the country, and the protestors were arrested by police. This time, the students were not supported by workers, but two years later, when in December 1970 the army fired at the protesting people in Gdańsk, Gdynia and Szczecin, those two social groups cooperated—and that helped end Gomułka.