One police officer has been killed and about 100 others injured in clashes outside Ukraine's parliament, Interior Minister Arsen Avakov said.
Police and nationalist protesters clashed outside the Supreme Rada on Monday afternoon after a controversial vote to give greater power to separatist regions in the east.
At least some of the officers were injured when an object thrown from the crowd of protesters exploded.
No injuries were immediately reported among the 100 demonstrators, most of them members of Svoboda, a nationalist party that holds only a handful of seats in parliament.
A military conflict between government troops and Russia-backed separatists has been raging since April 2014, claiming more than 6,800 lives.
Devolving powers to the regions was a condition of a truce signed in Minsk in February aimed at ending the fighting. But some Ukrainians oppose changing the constitution, saying that it would threaten the country's sovereignty and independence.
A total of 265 deputies in the 450-seat parliament gave preliminary approval today to the changes proposed by Ukrainian President Petro Poroshenko. Three parties that are part of the majority coalition in parliament, however, opposed the constitutional changes.
"This is not a road to peace and not a road to decentralisation," said the leader of one of those parties, former prime minister Yulia Tymoshenko. "This is the diametrically opposite process, which will lead to the loss of new territories."
Parliament speaker Vladimir Groisman denied that the changes would lead to the loss of the Donetsk region, where there have been clashes with separatists. "There is no hint of federalism. Ukraine was, is and will be a unified state," he said.
A final vote on the constitutional changes is to be held during parliament's autumn session, which begins tomorrow.