Ukrainian nationalists and security forces clashed outside the country's parliament building in Kyiv, injuring scores.

A National Guardsman died after being hit with shrapnel, apparently from a hand grenade.

The fighting erupted on August 31 as lawmakers considered legislation aimed at expanding autonomy for territories held by pro-Russian separatists in eastern Ukraine.

It was the worst violence to hit the Ukrainian capital since the February 2014 Euromaidan protests that ousted President Viktor Yanukovych.

Interior Minister Arsen Avakov said the guardsman died outside the parliament building where nationalists were demonstrating.At least four "life threatening injuries" were caused by what officials said was a hand grenade thrown during the clashes.

Avakov said at least 122 people were injured, mostly security forces, including one officer who lost both of his feet as a result of the blast. Olha Bohomolets, an adviser to the president, said at least 119 people were injured.

President Petro Poroshenko called the violence a "stab in the back."

"It was an anti-Ukrainian act for which all of its organizers without exception -- all representatives of political forces -- should be severely punished," Poroshenko said in a televised address.

A BBC correspondent described hearing three stun grenades followed by a louder explosion.

Video footage showed more than a dozen riot troops limping after the explosion, and at least one plainclothes officer falling to the ground and being dragged away. Other footage showed bloodstains on the pavement and servicemen dragging away at least two other wounded security officers.

Kyiv police said a man suspected of throwing a hand grenade was arrested, and about 30 other protesters were arrested.

Earlier reports said security forces used batons and pepper spray against the demonstrators.

WATCH: At least one Ukrainian National Guard soldier was killed in clashes with demonstrators after a grenade explodes outside parliament in Kyiv.

Smoke bombs also were used, but it was not immediately clear whether those devices were thrown by police or by protesters.

After the largest explosion, protest organizers announced the end of the demonstration, saying that provocateurs might have infiltrated their ranks.

Kyiv Mayor Vitaliy Klitschko said in a statement that "all those involved in the provocation" outside of the parliament would be punished.

The violence erupted after parliament approved a draft constitutional amendment granting more autonomy to separatist-held territories in the eastern regions of Donetsk and Luhansk, where fighting has raged on and off since April 2014.

The bill, which was submitted by Poroshenko in accordance with February's Minsk cease-fire agreement, was backed by 265 of the 368 lawmakers present.

Before the vote, dozens of lawmakers from the Radical Party and Svoboda political alliance blocked the parliament's podium and chanted "Shame! Shame!"

The Svoboda bloc said the government was to blame for the clashes by pushing the legislation, which it called an act of “capitulation to the Kremlin.”

The amendment now goes to a second vote where it will need at least 300 votes from the 450-member legislature to pass.

With reporting by Reuters, AP, AFP, BBC, and Interfax