Ukrainian President Viktor Yanukovich appeared on Sunday to rule out any prospect of opposition leader Yulia Tymoshenko being freed, saying those responsible for signing a 2009 gas deal with Russia on "enslaving" terms should be punished.
Tymoshenko, in a statement from prison where she is serving a seven-year jail sentence, called on all opposition forces to unite to defeat Yanukovich's Regions Party in a parliamentary election next October.
Her jailing last October for abuse of office, linked to the 2009 gas contract she brokered as prime minister, has led to a crisis in relations between the ex-Soviet republic and the West.
The United States and the European Union say the trial was politically motivated; in December the EU withheld completion of agreements on political association and a free trade zone with Ukraine in protest over her jailing.
Tymoshenko's trial and conviction are widely seen as a settling of scores between rival groups in the ex-Soviet republic.
Tymoshenko was a key player in the "Orange Revolution" street protests in 2004-2005 which overturned Yanukovich's first bid for the presidency. He made a comeback and narrowly defeated her in a run-off vote in February 2010.
Despite the EU pressure, justice officials have opened fresh criminal cases against Tymoshenko and she has been moved from police detention in the capital, Kiev, to a remote prison camp in Kharkiv, some 500 km (310 miles) to the east.
The Ukrainian leadership says the 2009 gas deal saddled the country with an exorbitant price for gas and committed it to importing volumes of Russian gas it does not need.
"Ukraine has become hostage to enslaving gas agreeements ... which have caused the country huge losses, billions of losses. We have been left with a huge external debt. Those who, regardless of their office, pushed Ukraine to the abyss must bear responsibility before the Ukrainian people," Yanukovich told a ceremony on Sunday.
Several thousand opposition supporters used a rally in the centre of Kiev, marking the day when eastern Ukraine joined the west of the country, to protest at the government's policies and call for Tymoshenko to be freed.
In a statement read out to the crowd, Tymoshenko called on all opposition parties to unite in a single democratic opposition force. "One team - one victory - this is the only slogan with which the opposition should arm itself today," she said.
In what might prove to be a significant development in the coming months, several opposition parties later issued a statement saying they would unite to field a single candidate in single-mandate constituencies at next October's election.
These will account for half the seats in the Ukraine parliament, the other half coming from party list voting.
The popularity of Yanukovich's Regions Party is sagging at present, and a united opposition might make it difficult for the Regions Party to re-establish a stranglehold on parliament after the election.
Tymoshenko's daughter told Reuters last week that her mother's life was "at risk" in the Kharkiv jail where she is being held because of her deteriorating health.
She said her mother had been unable to get up unaided since early November because of recurring back pain.