Ukrainian opposition protesters on Tuesday attacked the party headquarters of embattled President Viktor Yanukovych as fierce clashes with police erupted again in Kiev for the first time in weeks.

Protesters briefly seized the party headquarters after several hundred attacked it with Molotov cocktails and smashed their way inside but later withdrew as smoke continued to billow from part of the building, an AFP reporter at the scene said.

It marked the first violent clashes since mid-January in the Ukrainian capital, which has been wracked by anti-government demonstrations since Yanukovych in November rejected an EU pact in favour of closer ties with historical master Russia.

Demonstrators turned their ire on Yanukovych's party after clashes broke out with riot police around the nearby building of the Ukrainian parliament, where some 20,000 mainly peaceful protesters had massed to demand legislators strip the president of a raft of powers.

Police fired rubber bullets and hurled smoke bombs and stun grenades at protesters who threw paving stones and set two trucks on fire trying to break through to the heavily-fortified parliament.

Ukraine's interior ministry said in a statement that three servicemen were injured after protesters directed a truck through the police ranks.

A photographer was injured by a stun grenade in the turmoil and taken to hospital, an AFP reporter said.

Demonstrators were calling on the Rada parliament -- where Yanukovych's party has a majority -- to vote on returning the country to its 2004 constitution, under which key powers would shift from the president to parliament.

The demonstrators had marched from Kiev's iconic Independence Square, where the opposition remains firmly entrenched in a sprawling tent city after nearly three months of protests against Yanukovych's rule.

Opposition leaders called on Yanukovych to give into their demands if he wanted to defuse the violence.

"The president of Ukraine must call early presidential and parliamentary elections. I am sure that this will reduce the temperature of society," former heavyweight boxer and opposition leader Vitali Klitschko said in televised comments.

"Do this, and this will be a way out. This will be a courageous step," Klitschko said.

The US ambassador on Twitter lamented the resumption of violence. "After weekend progress in Kyiv, sorry to see renewed violence," envoy Geoffrey Pyatt wrote. "Politics needs to happen in the Rada, not on the street."

Before the latest outbreak of violence on Tuesday, Ukraine appeared to be inching towards resolving its worst post-Soviet crisis that was sparked by Yanukovych's decision in November to reject an EU pact years in the making.

The pro-EU, anti-government protests have since snowballed into a titanic tug of war for Ukraine's future between Russia and the West.

On Monday the government granted an amnesty to those arrested in the protests after the opposition vacated Kiev's city hall and other administrative buildings it had been occupying.

Also on Monday, Klitschko held talks in Berlin with German Chancellor Angela Merkel, urging her to slap sanctions on Yanukovych and his financial backers in a bid to ratchet up pressure on the embattled leader.

During the meeting, which was also attended by opposition leader Arseniy Yatsenyuk, Klitschko asked Merkel for "financial assistance to overcome the crisis" in the beleaguered nation.

Merkel stressed that Germany and the EU would do everything to contribute towards a "positive outcome" to the turmoil, spokesman Steffen Seibert said in a statement, and called for lawmakers to push on with reforms.

The talks in Berlin came as Russia's Finance Minister Anton Siluanov vowed to release "this week" a $2 billion tranche of a $15 billion bailout package to Ukraine that Moscow had essentially frozen after the protests turned deadly last month.

So far only $3 billion of the bailout has been transferred to Ukraine's economy, which is struggling to recover from a recession and is being badly hit by capital flight and currency devaluation during its prolonged crisis.

In a concession to the protesters, Yanukovych has dismissed his unpopular government, but he has yet to appoint a new one and the opposition wants its members to be placed in key positions.