Muslims in the predominantly Catholic Alpine country first filed a request to build a mosque in the late 1960s while Slovenia was still part of the former Communist Yugoslavia.
The community finally received permission 15 years ago, but ran into opposition from right-wing politicians and groups, as well as financial troubles.
Construction, which began in 2013, cost some 34 million euros ($39 million), out of which 28 million euros were Qatari donations, according to Grabus.
Situated in a semi-industrial area of Ljubljana, the mosque, which can hold up to 1,400 people, constitutes the core of the six-building Islamic Cultural Center.