Ukraine protests: Kiev battle zone like a 'vision of hell' as opposition issues ...

A protestor throws a molotov cocktail at riot police in the centre of Kiev (Picture: Vasily Maximovv / AFP / Getty)Ten of thousands of protesters clashed again with police in the Ukrainian capital of Kiev as opposition leaders issued a stark ultimatum to president Viktor Yanukovych.

It comes after at least four protesters were killed , two of them reportedly shot, in a grim escalation of a two-month political crisis.

The opposition is demanding Yanukovych call early elections within 24 hours or face more unrest on the streets.

MORE: Streets of rage erupt in flamesA demonstrator throws a rock during clashes between protestors and police (Picture: AFP / Getty)

'You, Mr President, have the opportunity to resolve this issue,' opposition leader Vitali Klitschko told some 40,000 people who braved freezing temperatures on the streets of the capital.

'Early elections will change the situation without bloodshed and we will do everything to achieve that.'

Ukrainian opposition leader Vitali Klitschko was targeted with a dry chemical fire extinguisher by radical protesters (Picture: EPA)

Another opposition leader, Arseniy Yatsenyuk, declared: 'If tomorrow the president does not make a step forward, we will attack.'

Pictures show streets in central Kiev ablaze and covered with thick black smoke from burning tires as protesters continue to clash with riot police.

A demonstrator holds a chain and riot police shield (Picture: AFP / Getty)

The BBC's Daniel Sandford said: 'Central Kiev square is like a vision of hell'.

The protesters' deaths were the first since the largely peaceful protests started in November.

At least four activists were killed as Ukrainian police stormed protesters' barricades in Kiev (Picture: AFP / Getty)

It has fuelled fears that the demonstrations against the government, over its decision to shun the EU for closer ties to Moscow and over human rights violations, could turn more violent.