A third Ukrainian policeman has died from injuries suffered in street battles with protesters in Kiev, and more than 140 people are being treated in hospital, officials say.

The clashes flared outside parliament on Monday as demonstrators protested at legislation approved on first reading by lawmakers giving more autonomy to pro-Kremlin rebels in eastern Ukraine.

A member of Ukraine's National Guard died on Monday and interior minister Arsen Avakov said on Tuesday another had succumbed to his injuries.

"It's painful," he said on Twitter.

Authorities blamed ultra-nationalists for the unrest — the worst in Kiev since the popular bloody uprising that ousted Moscow-backed president Viktor Yanukovych in early 2014, unleashing a separatist insurgency in the industrial east.

"According to our latest data, 141 injured people remain in hospitals all over Kiev," police spokeswoman Oksana Blyshchyk said.

She said 131 of those were policemen and 10 of them were in serious condition.

President Petro Poroshenko, who branded the violence a "stab in the back", was planning to visit the wounded later on Tuesday.

He said the perpetrators deserved "severe" punishment.

Olga Bogomolets, an aide to Mr Poroshenko, said on her Facebook page the most severely wounded suffered injuries to the stomach, lungs and brain after a grenade exploded near the entrance to the parliament building.

Authorities blame ultra-nationalist party for violence

Authorities pointed the finger at activists from the ultra-nationalist Svoboda party for the violence but they denied responsibility.

Authorities blame ultra-nationalists for the unrest which has left two dead and at least 140 people in hospital. ( Reuters: Valentyn Ogirenko )

The Kiev police spokeswoman said 18 people including a member of Svoboda's paramilitary wing accused of firing off the grenade were still in detention.

The clashes came as a shock to Ukrainians who have been struggling with the pro-Russian insurgency in the east of the ex-Soviet country since April last year.

"The constitution blown to pieces," screamed a front page headline in pro-Moscow daily newspaper Segodnya (Today).

"Bloody amendments," wrote pro-Kiev daily Ukraina Moloda (Young Ukraine).

Western countries voiced deep concern at the bloodshed.

AFP