(Adds activists saying toll may be higher)
DONETSK, Ukraine, March 13 (Reuters) - A young man was stabbed to death and more than a dozen people were in hospital on Thursday after rival Ukrainian demonstrators clashed in the mainly Russian-speaking eastern city of Donetsk, medical officials said.
In the worst violence since last month's overthrow of Ukraine's Moscow-backed president, hundreds of people waving Russian flags and chanting for Russian President Vladimir Putin scuffled on the central Lenin Square with demonstrators flying Ukrainian flags and condemning Russia's takeover of Crimea.
By late evening, the streets were quiet again.
The local health authority said a 22-year-old local man died of a knife wound and 15 others were being treated in hospital. Organisers of the pro-EU rally, which also denounced Russia's takeover of Crimea, said the dead man was from their group.
Journalists saw police try and fail at times to keep the two sides apart as the rival crowds threw smoke bombs and other missiles. Scattered fights broke out around the square and nearby streets as the demonstrators began to disperse.
Activists opposed to Russia's move in Crimea later said that a further two demonstrators had died, but officials said they had no such information.
Donetsk is the main city in the Ukraine's Donbass industrial heartland, close to the Russian border. Ukrainian officials have accused Russia of supporting groups in the area which favour rule from Moscow and of sending militants over the border.
Three weeks ago, about 100 people were killed in the capital Kiev, many of them by police gunfire, in clashes that prompted the flight of Moscow-backed president Viktor Yanukovich. He triggered the protests in November by rejecting a trade deal with the European Union in favour of economic aid from Russia. (Reporting by Lina Kushch; Writing by Alastair Macdonald; Editing by Ron Popeski)