* President Yanukovich to return after four days sick
* Thousands of protesters gather in Kiev
* Opposition leaders say West ready to support them
Ukraine's embattled president announced he would return to work after four days' sick leave, as protesters filled Kiev's main square on Sunday demanding he give up power.
Opposition leaders, addressing the crowd on their return home from meeting European and U.S. officials, said they hoped for international mediation in negotiations with the government and for constitutional change to limit presidential power.
Calling for a complete change of leadership after weeks of crisis that have divided the country and set the West against Yanukovich's Russian allies, opposition figures who attended a security conference in Munich told supporters they would secure international economic aid if they were able to take power.
Yanukovich, who angered opponents in November by spurning a trade pact with the European Union and turning instead to Moscow for financial support, announced on Thursday he was on sick leave and has not been seen in public since.
Critics saw in that a tactic to deflect pressure for political compromise. On Friday, he signed legislation revoking unpopular new restrictions on protest meetings that has, however, failed to appease opponents who are demanding the release of dozens of people arrested in recent weeks.
On Sunday, a presidential statement said Yanukovich planned to return to work on Monday after an acute respiratory infection: "After undergoing required treatment, the president of Ukraine feels well and his health is satisfactory," it quoted a state medical official, Oleksandr Orda, as saying.
On the capital's Independence Square, focus of a sprawling, barricaded protest camp throughout the winter, thousands of people gathered to listen to opposition leaders despite a freezing wind and Arctic temperatures which helped keep attendance well below those of major rallies in recent weeks.
Vitaly Klitschko, a former world champion heavyweight boxer-turned-politician, said they had discussed with senior Western officials in Munich bringing in international mediators in talks with the Ukrainian authorities.
"The democratic world has understood that there is no trust in the Yanukovich regime," he told the crowd.
"So we spoke about international mediation in negotiations with Yanukovich, so that afterward there will no differing interpretations of obligations."
Arseny Yatsenyuk of the Batkivshchyna party, who turned down an offer last week from Yanukovich to become prime minister, called on the authorities to free 116 prisoners. The president signed a law allowing protesters to be set free - but only once demonstrators stop occupying public buildings.
Yatsenyuk also renewed a call for a release of his party's leader, former prime minister Yulia Tymoshenko, whose freedom has also been a demand of the European Union. And he said there should be an international inquiry into "the criminal regime", held under the auspices of the Council of Europe.
"We told our Western partners we need real financial help to get the country out of crisis," Yatsenyuk said.
"But when we say 'we', we mean the Ukrainian people - not a penny must go to the Yanukovich regime.
"They are ready to help the Ukrainian people. But we must restore popular authority," he added. "Parliament must amend the constitution and get rid of the dictatorial powers of the president."
EU WANTS ELECTIONS
Some opposition leaders have urged the European Union, whose foreign policy chief Catherine Ashton is due in Kiev on Tuesday, to impose sanctions to hurt the business and financial interests of the president and his leading supporters. However, few European governments see that as worthwhile at present.
Ashton said in Munich on Sunday: "It's really important first and foremost that the violence in Ukraine stops, and that they move to a constructive dialogue, look at the issues of constitutional reform, the role of parliament and moving forward so that people can have faith in the transparency and openness of the process that will eventually of course lead them to free and fair elections in the future."
A senior U.S. State Department official is also due to visit Kiev this week.
At least six people were killed during protests last month and some leading figures have warned of "civil war". Since the resignation a week ago of Yanukovich's prime minister, the government has been operating in an interim capacity and some opposition leaders say they fear the president might impose a state of emergency or military rule on the country.
"Ukraine desperately needs a Marshall Plan and not martial law," Yatsenyuk said in Munich on Saturday, referring to the massive U.S. aid to western Europe after World War Two.
Russia's foreign minister, also in Munich, criticised the West for trying to force Ukraine into forging closer ties.
President Barack Obama said the United States was working to promote negotiations between Ukraine's rival factions.
Criticising the now abandoned attempts by the government to restrict rights to protest, Obama said in a video message distributed online by the U.S. embassy in Kiev on Sunday:
"There has to be a way to restructure the Ukrainian government in a way that allows the voices of the opposition, and those folks on the streets, to be heard in preparation for some sort of democratic process that creates a government with greater legitimacy and unity.
"That's going to be challenging but we're trying to help on the negotiations on that."
He said close historic and commercial ties to Moscow, which ruled Ukraine under the tsars and in the Soviet era, "don't need to be sacrificed", but added: "The people of Ukraine clearly are looking to Europe and the West as a partner in a more free market-based economy."