(KIEV) - Ukraine's opposition plan a mysterious "peaceful offensive" at a new mass protest on Sunday to put pressure on authorities, as negotiations to exit the crisis appear to have stalled.

Anti-government protesters have occupied Kiev's central Independence Square for almost three months after President Viktor Yanukovych rejected a key EU trade pact in favour of closer ties with Russia.

And while the movement shows no sign of dying down, with protesters rebuilding and fortifying barricades to keep riot police out of their anti-government enclave, impatience is gaining hold.

"The main issue which will be addressed (at the Sunday protest) will be a peaceful offensive by activists so that the demonstrators' requirements be fulfilled," the party of jailed opposition icon Yulia Tymoshenko said in a statement late Thursday.

It did not detail what form this "peaceful offensive" would take.

The crisis in Ukraine continued to stoke wider, geopolitical tensions between Russia and Western countries Friday, with Moscow accusing the European Union of trying to extend its "sphere of influence" in the country.

The mass demonstration in the Ukrainian capital will be the 11th since the protest movement erupted in November, and will kick off at midday (1000 GMT) at the heart of the sprawling, barricaded tent city on Independence Square.

It will come a day ahead of the much-anticipated Monday deadline for an amnesty offer made by Yanukovych. Also on Monday, Ukraine's opposition leaders are expected to meet with German Chancellor Angela Merkel in Berlin.

Yanukovych started negotiating with opposition members after protests at the end of January turned violent, leaving four dead and more than 500 injured in unrest that shocked the country.

Moscow-friendly Prime Minister Mykola Azarov resigned, and Yanukovych offered to free all people detained since the protests began in return for demonstrators evacuating official buildings they are currently occupying.

Kiev's prosecutor has said the offer runs out Monday.

- No end in sight -

On Friday, the headquarters of the protest movement announced that all 243 people detained had been released from custody, Interfax-Ukraine news agency said, though many have instead been put under house arrest and still face serious charges that can carry up to 15 years in prison.

"We're not leaving this place until the last prisoner of this regime is freed," said Ruslan Andreiko, the "commander" of the occupied city council, where 600 to 700 protesters have been camping out, adorning the building with photos and satirical pictures.

"I believe the only condition for people to free the Maidan (Independence Square) is the resignation of President Yanukovych, followed by early presidential elections," he said, as protesters with various ailments consulted volunteer doctors nearby.

The city council is run with military precision, kept clean by countless volunteers who also work in the kitchen or at a makeshift pharmacy to dole out donated medicine, and these show no sign of wanting to move out.

Apart from the release of detainees, the opposition has also demanded that a new pro-West government be established -- but this too has yet to be announced.

Arseniy Yatsenyuk, an opposition leader who has been offered the premiership, has said he may accept if other opposition members get key government positions.

He also wants the constitution to be amended to reduce presidential powers in favour of parliament and government, but debate on this reform does not seem to be progressing.

Outside the country, the war of words between Russia and the European Union continued on Friday.

Foreign Minister Sergei Lavrov publicly rebuked the European Union at a joint press conference with his visiting German counterpart Frank-Walter Steinmeier.

"Pressuring Ukraine in one direction, while warning that it faces an 'either-or' choice -- either the EU or Russia -- is essentially trying to create a sphere of influence," he said in comments rejected by Steinmeier.

"This is not a geopolitical chess game taking place in Ukraine. We have to allow the Ukrainians to find their own path," Germany's top diplomat said.