"At least three people have died. All of them had gunshot wounds," said Oleg Musiy, head of a medical team that runs first-aid centres for protesters.
Another medic at the opposition-run field hospital told AFP that most of the injuries were caused by stun grenades.
Thirty people were in a serious condition, and some had suffered head injuries, he said. One person had to have a hand amputated.
Police said 47 security officers had been hurt in the clashes, with five sustaining bullet wounds.