Reuters TV footage from the scene showed members of Ukrainian police and protesters spraying tear gas at each other. The protesters attacked with long sticks and threw car tires and stones while the policemen barely fought back.
Interior minister Denys Monastyrskiy said eight policemen were injured during the clashes and a criminal probe had been launched. He wrote on Facebook that the clashes began when the police tried to search protesters on their approach to the president's office but they resisted.
"The Constitution guarantees the right for peaceful protests. Everyone who had undergone basic checks would have been allowed to enter the square in front of the President's office," he said.
The right-wing National Corps party organized the rally to protest against a plan known as the Steinmeier formula, which provides special status for the Donbass region, controlled by pro-Russian separatists.
The group said the violence was provoked by police officers when they blocked access to the building, and some protesters got injuries as a result.
"This regime… is in fact anti-Ukrainian," Andriy Biletsky, a party leader, was quoted by the website as telling his supporters.