Ukrainian riot police moved on Wednesday into part of the Kiev square where thousands of protesters and police have been engaging in violent street battles for four days.

Protesters were herded back around a stage in the centre of the square, from which a singer urged police: "Do not harm us!", Reuters reported.

Some held their mobile phones in the air like candles and sang the national anthem.

Several hundred demonstrators remained camped in the square, as police did not take further actions against them.

Ukrainian opposition leaders have issued an ultimatum to President Viktor Yanukovych, saying that protesters will be "on the attack" in the capital Kiev, if the government refuses to call snap elections.

Protest leader Vitali Klitschko said police were preparing to clear demonstrators out of the main protest encampment at Maidan (or Independence Square).

"We must do all we can to stop them clearing us out," he told demonstrators.

Another opposition leader, Arseniy Yatseniuk, said the government had 24 hours to respond to the protesters' demands.

"If this does not happen, we will march forward together. If it's a bullet to the head, then it's a bullet to the head," he declared.

Prime Minister Mykola Azarov said compromises "might be possible", but the opposition should avoid ultimatums.

The protest, which began in a peaceful way, turned violent as demonstrators set several police buses on fire, used Motolov cocktails and beat officers with baseball bats.

The uprising culminated in the killing of two demonstrators by police on Wednesday.

Ukraine's Radio Liberty reported the death of a third activist on Wednesday evening.

The escalating violence has caused all sites to warn of a possible civil war in Ukraine.

The protest was sparked following President Viktor Yanukovich's rejection of a trade and investment deal with the European Union. The demonstrations increased after Yanukovich reached a deal with his Russian counterpart Vladimir Putin.

While Russian Foreign Minister Sergei Lavrov blamed outside influences for fuelling the violence, Ukrainian politician Arseniy Yatsenyuk argued that the shift in protests tactics has been caused by Yanukovych's refusal to listen to the protesters' demands.