KIEV, Ukraine -- As lawmakers took up a measure to give greater powers to separatists in eastern Ukraine, nationalist protesters clashed with police outside parliament on Monday, and the Interior Ministry said one officer was killed in a grenade blast and more than 100 were wounded.
It was the worst violence in the capital since the government took power in February 2014.
The decentralization of power was a condition demanded by Russia for a truce signed in Minsk in February aimed at ending the fighting between Ukrainian government troops and Russia-backed separatists that has left more than 6,800 dead since April 2014.
But Ukrainian nationalists strongly oppose changing the constitution, saying that would threaten the country's sovereignty and independence.
In a televised address, Ukrainian President Petro Poroshenko called decentralization "a difficult but a logical step toward peace," and insisted that it would not grant autonomy to the rebels in Donetsk and Luhansk.
The measure won preliminary approval on Monday with 265 deputies in the 450-seat parliament voting for it.
But three parties that are part of the majority coalition in parliament refused to give their support, showing the difficulty that Poroshenko faces even within his own pro-Western camp in fulfilling the peace agreement.
When the decentralization bill comes up for final approval, he will need to get at least 300 votes as required for amending the constitution.
"This is not a road to peace and not a road to decentralization," said the leader of one of those dissenting parties, former Prime Minister Yulia Tymoshenko. "This is the diametrically opposite process, which will lead to the loss of new territories."
The officer who was killed in the clashes on Monday was a 25-year-old conscript, Interior Minister Arsen Avakov told reporters. He said 122 people were hospitalized - most of them officers, but also some Ukrainian journalists and two French reporters.
No injuries were reported among the several hundred protesters, including 100 die-hard activists, most of whom are members of Svoboda, a nationalist party that holds only a handful of seats in parliament. The protesters were carrying sticks and truncheons. Some of them were masked.
Avakov said that about 30 people have been detained, including the protester who threw the grenade, who he identified as a Svoboda member who fought in the east in one of the volunteer battalions that are loosely controlled by the government.
Poroshenko described the violence outside parliament as a "stab in the back" and pledged to prosecute "all political leaders" who were behind the clashes.
He said Monday's vote confirmed Ukraine's "position as a trusted partner which fulfills its international obligations" and said the country risks losing the support of the West and being left "alone with the aggressor" if it fails to meet the conditions of the truce.
The Minsk peace agreement was negotiated with the leaders of Germany and France, as well as with Russian President Vladimir Putin.
While Ukrainian nationalists fear the decentralization bill would incite separatism, Moscow and the Russia-backed rebels say it doesn't give the regions sufficient powers and falls short of the pledges Kiev made in Minsk.
A final vote on the constitutional changes will be held during parliament's fall session, which begins on Tuesday. No specific date has yet been set.
Prime Minister Arseniy Yatsenyuk, in a live address on television, denounced the violence, saying the right-wing protesters were "worse" than the separatist rebels because they were destroying the country from within "under the guise of patriotism." He called for life imprisonment for the protester who threw the grenade.
"The cynicism of this crime lies in the fact that, while the Russian federation and its bandits are trying and failing to destroy the Ukrainian state on the eastern front, the so-called pro-Ukrainian political forces are trying to open another front in the country's midst."
He called on all Ukrainian political parties to rally around the government and to condemn the violence.
Avakov blamed the clashes on the Svoboda party, which polled less than 5 percent in last year's parliamentary election, and its leader, Oleg Tyahnybok, who stood side by side with the interior minister during the anti-government protests that toppled then-president Viktor Yanukovych in February 2014.
"No political differences can justify what you did outside the Rada today," Avakov said, referring to the parliament.
Svoboda blamed the government, saying that it "provoked Ukrainians to protest" by presenting a bill that it called tantamount to "capitulation to the Kremlin."
Kremlin spokesman Dmitry Peskov, in comments to Russian news agencies, voiced Moscow's concern about the clashes in Kiev, but wouldn't comment on the bill.
European Union foreign policy chief Federica Mogherini called the clashes "worrying" and said the vote "will facilitate the implementation of the Minsk agreements."
In Berlin, German Chancellor Angela Merkel said she's open to holding a new summit with the leaders of Russia, Ukraine and France on the settlement in eastern Ukraine.