"The fortress Europe is unsustainable .... enlargement must continue," Slovene President Janez Drnovsek said.
Czech President Vaclav Klaus said it would be a tragic mistake to stop the enlargement process due to the Union's internal problems.
Polish head of state Aleksander Kwasniewski said the EU must express solidarity with and openness towards the countries of southern and eastern Europe.
Commenting on failed referendums on the European constitution in some EU member states, Slovene President Janez Drnovsek said those countries failed to explain to their citizens what the EU enlargement really meant and warned that this would lead to a Europe of divisions, namely a Europe of wellbeing on one side and a Europe of poverty on the other.
Italian President Carlo Azeglio Ciampi said the completion of the European integration process was the EU's fundamental objective and stressed that the rejection of the Constitution in some countries was not a sign of dismissing the enlargement process but a wish to ensure a better management of the EU.
The Czech president said barriers within the EU needed to be removed so as to ensure a better flow of people, commodities and capital.
The Polish president called for the inclusion of Ukraine, Moldova and Georgia in the European integration process, stressing that the said countries should be granted prospects of full membership, which Slovene President Drnovsek endorsed.
Albanian president Alfred Moisiu said his country had been investing efforts for over a decade into achieving EU economic, legal and political standards, and underlined that Kosovo must not be left outside the EU borders.
Moldova's President Vladimir Voronin confirmed his country's aspiration to gain EU membership and said the authorities in Moldova were carrying out the necessary reforms so as to adjust to the criteria for European integration processes.
In their speeches, the presidents hailed the start of Croatia's EU entry talks.
The plenary session will continue after a break with speeches by another eight presidents.