Some fundamental human rights and freedoms are sometimes restricted in the name of a successful fight against terror, Mesic said, adding he was aware that sometimes this was inevitable.
He went on to say that democracy proved its value by preserving its key elements, and urged affirming democratic values with the awareness that their restriction would mean that terrorism was starting to win.
Mesic reiterated his statement that the fight against terrorism must include combating terrorists as well as its roots.
He said the contribution of the UN and of each country individually could be immeasurable.
The conference pooled more than 200 former and current government officials, heads of state and presidents of international organisations.
Today's closing speeches were delivered by NATO Secretary-General Jaap de Hoop Scheffer, European Commission President Jose Manuel Barroso, Luxembourg Prime Minister Jean-Claude Juncker, European Parliament Speaker Josep Borrell, and King Mohammed VI of Morocco, among others.
The conference is being held under tight security measures, including 17,000 police. It ends tomorrow with a commemoration for the victims of the 11 March 2004 Madrid terrorist attack.