Poland and other central European nations are marking the 15th anniversary of their membership in the European Union with a leaders' summit in Warsaw.

Besides Poland, the other countries that joined the EU on May 1, 2004 were Cyprus, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Slovakia and Slovenia. The joining of the 10 countries was the largest single expansion in the bloc's history.

Polish Prime Minister Mateusz Morawiecki is hosting the government heads and officials at Warsaw's Royal Castle Wednesday. A declaration they signed says members should take decisions jointly and in the spirit of good cooperation.

Leaders from Bulgaria, Romania and Croatia, which joined the EU later, were also present.

Hundreds of far-right critics of the EU protested outside the summit's venue.