"Congratulations Croatia! As midnight struck, your country crossed an important threshold. It will change the life of this nation for good. You have always been Europeans - and you are now members of the Union. On behalf of the leaders of the 27 member states, many of whom are here, I welcome you wholeheartedly," Van Rompuy said at a ceremony in Zagreb's main square Trg Bana Jelacica.

"As of tomorrow your representatives will sit side-by-side with all other member states. Together shaping decisions for the benefit of all citizens across the Union - from Zagreb to Zaragoza, from Split to Stockholm," he said.

Van Rompuy said that Croatia's entry into the EU would also be a milestone for the region. "In the recent history of the Western Balkans, your country's entry marks a milestone toward a shared future: living together, in peace and prosperity."

"I'm proud to be here tonight for this historic moment," he said, praising Croatia for courageously overcoming hurdles in its relations with Slovenia and with Bosnia and Herzegovina, and expressing hope that more positive steps would follow in the region.

"Following the same path, Belgrade and Pristina have also reached a historic agreement. I am confident that more positive steps will follow. As a trailblazer for the region, your country, now as a member state, has a vitally important role to play," the European Council President said.