* Photographer sees at least 10 wounded
* Pro-Russian separatists armed with baseball bats (Updates casualties)
By Maria Tsvetkova
DONETSK, Ukraine, April 28 (Reuters) - Pro-Russian separatists armed with baseball bats attacked a rally in support of Ukrainian unity in the separatist-held city of Donetsk on Monday, amid rising tension and disorder in the country's east.
A Reuters photographer saw at least 10 people with head wounds after dozens of men dressed in military fatigues tried to disrupt the rally with baseball bats, firecrackers and what appeared to be at least one stun grenade.
Donetsk is at the heart of an uprising in the east by mainly Russian-speakers trying to throw off rule from Kiev, following the February overthrow of Moscow-backed President Viktor Yanukovich and Russia's annexation of Crimea in March.
Diana Berg, an activist opposed to the uprising, said about 20 participants in the rally, which gathered around 2,000, had been injured, of whom 10 were being treated in hospital.
Berg said most had suffered head wounds from stones or baseball bats. A Reuters reporter said some separatists were also among the injured.
Protesters waved Ukrainian flags and chanted "Donetsk is Ukraine!" and "Putin No!". They quickly dispersed after the violence.
Nikolai Solntsev, a spokesman for the self-proclaimed "People's Republic of Donetsk", blamed the violence on "emotions", and accused the pro-Ukrainian protesters of provocation.
He called the separatists "patriots of the city of Donetsk".
"All of them believe they need to defend their land," he told Reuters.
Elsewhere in the east, armed men seized public buildings on Monday in the town of Kostyantynivka, close to the rebel military stronghold of Slaviansk, where European military observers have been held captive since Friday. (Additional reporting by Marko Djurica; Writing by Matt Robinson; Editing by Andrew Roche)