Protesters throwing stones and paintballs disrupted the opening of an "anti-Islamification" conference Friday organized by a right-wing German nationalist group that opposes the building of a large new mosque.

Some 100 protesters gathered outside city hall in Cologne's borough of Rodenkirchen to prevent two leaders of the Pro-Cologne movement from entering the building where they were to hold a news conference.

Police moved in to build a protective ring around the two men, amid shouts of "Shame on you!" and "Get lost!" from the angry crowd. The nationalists were turned away at the door by a city official on orders from the mayor.

The group then sought refuge on one of the many rental boats that cruise the Rhine River, scrambling aboard with several journalists as protesters pelted the side of it with stones and paintballs.

The scuffle that left several windows broken, but no one was injured, police said. Eight people were detained.

After several hours on the river, the boat — named Moby Dick — docked, and dozens of protesters surrounded the nationalists, preventing them from moving further without police protection.

About 500 largely peaceful protesters also built a human chain around a site where the city has approved the building of a large, domed mosque — complete with two 55-meter-tall (177-foot-tall) minarets — in the city's heavily immigrant Ehrenfeld district. Construction is to begin by the end of the year.

Police prevented a planned bus tour by the nationalists through immigrant-heavy districts, arguing that it would have been provocative and could have led to unrest.

Pro-Cologne had billed its three-day conference as an attempt to build a "European, patriotic, populist right-wing movement" and invited members of nationalist parties from other European nations to attend.

The conference is to include a demonstration against the mosque on Saturday, and police say they expect several hundred nationalist supporters to attend, along with up to 10 times as many counter-demonstrators.

German government officials were swift to condemn the conference that also has drawn criticism from Islamic nations, including Iran.

Interior Ministry spokeswoman Gabriele Hermani said in Berlin that the "event organized by populists and extremists" was "damaging to the good cooperation between the city and its Muslim citizens."

Armin Laschet, the minister for integration in North Rhine-Westphalia state, where Cologne is located, noted that the hundreds of counter-demonstrators reflected citizens' disapproval of Pro-Cologne. "They are not a citizens' movement, it is a far-right group," Laschet said.

About 1 million people live in Cologne, more than 36 percent of them Turks or of Turkish descent.