The president of the new party will be Vesna Pusic of the People's Party, while Libra leader Jozo Rados will be vice-president.
Rados will also serve as vice-president of the joint Club of Deputies in Parliament, which will have 13 members in all, including two Libra deputies.
The HNS presidency will include four former members of Libra, and Libra members will also join other party bodies at national and local levels.
"The unification of the two parties will not remove the political risk, but will increase our strength and our chances of success," Pusic said after the unification.
Rados said that it was an important political event that would encourage further mergers of civic-oriented parties on the Croatian political scene.
Asked if he expected two other Liberal parties -- the Social Liberal Party (HSLS) and the Liberal Party (LS) -- to join the newly-formed party, Rados said he supported the announced merger of the two parties, but added that unification was not a simple step.
The HNS leaders said they expected to win between 10 and 15 per cent of votes in the forthcoming local elections.
Today's unification convention was attended by many guests from kindred parties, European organisations and representatives of most Croatian parliamentary parties, including the ruling Croatian Democratic Union (HDZ), the Social Democratic Party (SDP) and the Democratic Centre (DC), which said that the unification would help stabilise the political scene in Croatia.
This morning before the convention, the two parties' top bodies held separate sessions at which they supported the proposed unification almost unanimously. Seven of the 1,250 HNS delegates and three of Libra's 248 delegates voted against unification.