The European Coalition, comprising the Civic Platform formerly led by European Council President Donald Tusk, scored second with 39.1 percent, a poll by Ipsos pollster showed, according to Reuters.
Sunday's ballot is seen as a test for the popularity of PiS ahead of a national vote due this fall that will decide whether central Europe's largest economy can overcome its growing isolation within the EU.
Voters in the European Union went to polls earlier this week to choose members of the European Parliament (EP). The European Parliament is the legislative body of the European Union.
Both political parties and self-nominated candidates can take part in the elections. Only political parties and political organizations can nominate candidates in the Czech Republic, Denmark, Germany, Greece, the Netherlands and Sweden. In all other EU member states nominations may be submitted if a candidate receives the required number of signatures or electors.
In the vast majority of countries, elections to the European Parliament are held in a single constituency. However, in some states — in Belgium, Ireland, Italy, Poland and the United Kingdom — the territory is divided into a number of regional constituencies.
The last elections to the European Parliament were held from 22-25 May 2014, and became the largest transnational elections. The major European political parties for the first time nominated their candidates for president of the European Commission, the executive body of the European Union.