By Marko Kovac BBC, Zagreb

Motorists honked their horns as they drove around the building in the capital, Zagreb.

It is celebration time for HDZ supporters

There were tears and disbelief among supporters of the centre-left coalition led by Prime Minister Ivica Racan.

Although ballots from Sunday's poll are still being counted, it is clear that Croatia has swung back to the right in its fourth elections since it gained independence in 1991.

Many identify the HDZ with the late authoritarian ruler Franjo Tudjman.

But while celebrating victory, the HDZ's moderate president Ivo Sanader told the BBC that he was looking to the future, not the past.

"The party has been reformed under my leadership and the election outcome is the result of that," he said, making clear he wanted to leave the party's nationalistic past behind.

Knocking on EU's door

Once run by hardline nationalists, the HDZ now pledges to promote the return of Serb refugees who fled Croatia during the conflict in the 1990s.

Sanader insists his party has changed its spots

The party also promises full co-operation with the United Nations International War Crimes Tribunal in The Hague.

This is an unpopular move, as many Croats see indicted general Ante Gotovina - currently at large - as a national hero.

The party's moderate messages surprised everyone, as the party's conservative wing remains strong.

The HDZ failed to co-operate with the centre-left government on these key issues.

If the outcome of the elections is confirmed, the HDZ will have to prove it is ready to commit to these basic criteria - which will open the door to further negotiation on European Union membership.

Brussels will now take a close look at the HDZ's policies. Croatia is due to start negotiations on EU membership next year. In the meantime, the HDZ must show that its words can be turned into reality.

Looking for partners

As we visited the HDZ's headquarters on Monday, party officials seemed to be getting down to the business of putting together a ruling coalition.

The result did not give them an absolute majority in parliament.

In their fancy offices in Zagreb, decorated with Franjo Tudjman's pictures, Mr Sanader's mobile phone rang constantly.

Some of the calls probably came from future partners, among them the conservative farmers' party, the HSS. It was part of the outgoing coalition, but now seems to be shifting sides.

"This is the most natural coalition, and the most probable one," said influential political analyst Davor Gjenero.

With the HSS as a partner, the HDZ would have a majority.

The next parliamentary term could prove crucial for Croatia's reforms, as well as for its integration into the EU.