The event was attended by some 20,000 Muslims from Croatia and neighboring European countries.
The inauguration event caps years of work in the mosque that was first suggested in 1968.
Yet, construction of the center, designed by a prominent Croatian, the late Dusan Dzamonja, began only in October 2009 after getting the finance from donations of Qatar’s emir Sheikh Hamad bin Khalifa al-Thani.
The project was estimated to have cost about 10 million euros ($13 million).
The Islamic center consists of a large silver mosque with a 23-meter (75-feet) high minaret on a hill overseeing Kvarner bay.
The complex of the Islamic Center also includes a conference room, dining room, guest house, classrooms for young people, cafeteria, basketball and soccer courts, a parking lot and multi-purpose halls.
Qatari Minister of Endowments and Islamic Affairs Ghaith bin Mubarak Al-Kuwari and a number of Croatia's top officials and Catholic church representatives attended the ceremony.
"Multiculturalism and diversity are among basic values on which the European Union is built," the head of EU delegation here, Paul Vandoren, said, a reminder that Croatia was set to join the bloc on July 1.
The celebration was also attended by Muslims from several European countries.
"For Muslims Croatia is good place to live in and could serve as an example to other European countries," said the Muslim member of neighboring Bosnia's tripartite presidency, Bakir Izetbegovic.
"Today's event sends good signals to the whole world and notably to the region" of the Balkans, ravaged by inter-ethnic wars in the 1990s, he said.
Aziz Emini, an ethnic Albanian from Macedonia now living in Germany, said his family "came to witness real peace among religions."
"Croatia, which tomorrow will enter the Europan Union, could serve as an example of integration of Muslims in Europe," the 47-year-old baker said.
Muslims make only some 1.5 percent of Croatia's 4.2 million inhabitants. Almost 87 percent of the population are Roman Catholics.
Out of Croatia's 63,000 Muslims around 10,000 live in the Rijeka region.The mosque in Rijeka is the third one built in Croatia.
The biggest mosque is in the capital Zagreb, while the eastern town of Gunja, on the border with Bosnia, hosts another one.
Source: OnIslam