Ukraine's pro-European opposition sought last minute changes to a deal with Russian-backed President Viktor Yanukovich intended to resolve the former Soviet republic's political crisis, but EU mediators said they still expect an agreement on Friday.

The German and Polish foreign ministers were in Kiev to try to broker an end to the worst violence since Soviet days amid a stand-off between riot police and anti-government protesters who have occupied a central square for nearly three months.

The sprawling nation of 46 million with a shattered economy and endemic corruption is at the center of a geopolitical tug-of-war between Russia and the European Union.

Diplomats said a compromise drafted in all-night negotiations involved appointing a transitional government, with a reformed constitution by September reducing presidential powers, and fresh elections by the end of the year.

Whether such a gradual transition would be acceptable to grassroots activists who want Yanukovich out now was unclear.

"This is just another piece of paper. We will not leave the barricades until Yanukovich steps down. That's all people want," said Anton Solovyov, 28, an IT worker protesting in the central square.

A senior EU diplomat said the president and opposition leaders were expected to sign the deal on Friday, even though the opposition still wanted some changes.

Earlier, police said in a statement that anti-government militants fired on security forces near the central Independence Square, scene of a three-month-old protest vigil.

However, there was no independent confirmation of such an incident and no report of casualties.

The square, known as Maidan or "Euro-Maidan", appeared peaceful, with thousands of demonstrators chanting anti-government slogans interspersed with patriotic singing.

After the visiting ministers suspended talks at dawn, the presidential press service said an agreement with the opposition would be signed at noon (1000 GMT) but it gave no details. That time passed with no word of a signing.