Hundreds of supporters of patriotic groups across Ukraine descended on the city to protest against the re-erection of a statue that was removed eighty years ago.
The nationalists describe themselves as heirs of Cossacks. They accuse Catherine the Great of colonising Ukraine, and say her monument is an affront and a threat to Urkainian independence.
Nationalist leader Ihor Vardanets says "honouring a woman who enslaved Ukrainian people" isn't right.
"She made our country a minor part of Russia, and turned Ukrainians into serfs," he said.
Many residents of Odessa, however, have welcomed the statue as a step towards reviving the city’s historic past.
Lieutenant General Sergey Elistratov, the leader of another group describing itself as Cossack, described the protests as hooliganism.
“They broke fences, washed their shoes in the fountain at the Pushkin monument. They are vandals," he said.
"Today we are here to defend law and order, to defend our city”.
Odessa was officially founded in 1794 as a Russian naval fortress.
The first monument to Catherine was removed from the city by Soviet authorities in the 1920s.
As part of a project to revamp the city centre, Odessa Council returned the statue of the Russian Empress back to the square that bears her name.