Ukraine’s president announced that he would return to work after four days’ sick leave, but protesters filled Kiev’s main square on Sunday demanding that Viktor Yanukovych give up power.

Opposition leaders, addressing the crowd on their return from meeting European and U.S. officials in Munich, said they hoped for international mediation in negotiations.

“The democratic world has understood that there is no trust in the Yanukovych regime,” Vitali Klitschko, one of the opposition leaders, told the crowd. “So we spoke about international mediation in negotiations with Yanukovych, so that afterward there will no differing interpretations of obligations.”

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. But few European governments see that as worthwhile at present.

Yanukovych, who angered opponents in November by spurning a trade pact with the E.U. and turning instead to Moscow, announced Thursday that he was on sick leave. Critics saw in that a tactic to deflect pressure for political compromise.

On Sunday, a presidential statement said he would return to work on Monday after an acute respiratory infection.