The centre-right party was yesterday predicted to hold the capital Warsaw as well as win Gdansk, Wroclaw and Poznan in the second round of a highly-contentious vote.
The PO mayor of Warsaw, Hanna Gronkiewicz-Waltz, was elected for a record third time taking 56.8 per cent of the vote against her conservative rival Jacek Sasin.
An Ipsos poll had the party leading also in Gdansk on the Baltic, Poznan in the west and Wroclaw in the southwest.
The elections - which came six months before Poland chooses its next president and a year before legislative elections - were seen as a political barometer following the departure two months ago of centrist prime minister Donald Tusk to become president of the European Council.
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The technical problems in the November 16 first round led to results being delayed by a week.
Those results confirmed exit poll predictions of a narrow lead in regional councils for the opposition conservative Law and Justice (PiS) party, their first election win in nine years.
PiS leader Jaroslaw Kaczynski had threatened to challenge election "fraud" in the courts and has called for a demonstration in Warsaw for December 13, a highly symbolic date for Poles, when communist leader General Wojciech Jaruzelski cracked down on the Solidarity union.