One of the president's main foes described his offer as a "poisoned" attempt to divide the opposition and kill off mass protests. The demonstrations erupted late last year when Mr Yanukovich ditched landmark agreements with the European Union and opted instead for closer ties with Russia.
Opposition leaders said they would press for more concessions, including early elections, setting the stage for a tough political battle when parliament meets for a special session tomorrow.
The two-month standoff has sparked the worst violence in Ukraine since it won independence in 1991, as the Soviet Union collapsed. At least six people have been killed, according to the prosecutor's office and medics.
For the opposition, accepting Mr Yanukovich's offer to serve under him in a revamped government carries the risk of breaking faith with thousands of peaceful demonstrators, as well as alienating more radical protesters.
Yesterday a few thousand protesters tried to storm an ornate cultural centre where hundreds of security forces personnel were gathered in central Kiev.