The Round Table talks.

The Round Table Talks and the Fall of the Communist Dictatorship
In 1989, faced with an increasing economic crisis and under the influence of perestroika in the Soviet Union led by Mikhail Gorbachev, General Jaruzelski decided to open talks with the opposition. In March, the Round Table talks began, which resulted in the authorities agreeing to re-legalize Solidarity and to carry out partially free elections to parliament. Groups independent of the Communist Party could run for election to 35% of the seats in the lower house and 100% of the seats in the Senate.
On June 4, elections were held that resulted in Solidarity winning all the seats they could in the Sejm and 99 out of 100 in the Senate.
The landslide victory of the opposition led to the appointment in September 1989 of the first non-communist government in Eastern Europe, led by Tadeusz Mazowiecki, one of the opposition leaders and an adviser to Solidarity.