Hundreds of spectators gathered at the city's Sukhbaatar Square Saturday to view the fossils, particularly one of a Tyrannosaurus that had gone on auction in New York on May of last year, Chinese news agency Xinhua reported.
The Mongolian government lodged a protest, claiming it had been illegally removed from the county and asking the U.S. government to return it.
U.S. authorities seized the fossil after filing charges against the U.S. importer, and returned it to Mongolia.
Many dinosaur fossils have been smuggled into Europe, North America and Asia, and more than 120 would be returned to their homeland in the near future, Mongolian officials said.
To protect the country's abundant dinosaur fossil resources, the government has decided to build a Mongolian dinosaur museum Tsedevdamba Oyungerel, Minister for Culture, Sports and Tourism, said at the exhibit in Ulan Bator.