Thousands are marching in the Polish capital to celebrate the nation's European Union membership ahead of key European Parliament elections.

EU Council President Donald Tusk and Poland's opposition leaders are among those marching in Warsaw. The pro-EU rally aims to underline that Poles want to be an important member in the 28-nation bloc, even though the current rightwing government is highly critical of the EU and its institutions.

Before the march, leaders from center-left parties called on the Poles to vote in May 23-26 elections, saying they are crucial for Poland's standing in the EU.

The elections are also seen as a test of support between the ruling party and the opposition, ahead of national parliament elections in the autumn.